The existence of two different verbs that mean “to be” SER and ESTAR is a very curious feature of Spanish and Portuguese. Since the verb “to be” (and its conjugated forms) is probably the most fundamental element of any language, your mind quite literally has to change to accommodate this distinction. It is worthwhile to learn Spanish just for the mind-expanding effect that one can experience upon gaining the full appreciation of this difference.
If I were to pick one cool grammar trick from each language with which I have some level of familiarity, I would say that for English it would be the ability to turn any noun into a verb. The fact that you can say “To greenlight a project” or “We are just dogfooding the phone at this point”  is just absolutely remarkable (and impossible in many other languages). For Russian, I would pick the liberation from from word-order. Since in Russian the grammatical roles of the words are expressed by their conjugated endings, you can put the words in a sentence in any order you wish, without breaking any rules or sounding odd — the words still “know” what their roles are. This flexibility is what makes the Russian language such good clay for poets. 
Where is this all going? Oh yes, I remember, for Spanish and Portuguese, I would pick the existence of SER/ESTAR dichotomy.
The usual explanation given in grammar books for when to use SER or ESTAR is very similar to this one from Cliffsnotes:
“Both the verb ser and the verb estar mean “to be” but each indicates a specific type of “being.” Ser is used to describe more permanent states of being and estar is generally used for more temporary states, but there are some exceptions.” 
Here are some examples that would conform to the rule quoted above:
Robert está ocupado. (ESTAR, temporary)
Robert is busy (at the moment)
Robert es tímido. (SER, permanent)
Robert is shy.
There are many adjectives that can reflect qualities that can be permanent or transient. To translate these into English we need to do more work since the English word we use will depend on the choice of SER or ESTAR.
Robert está aburrito. (ESTAR, temporary)
Robert is bored. (right now)
Robert es aburrito. (SER, permanent)
Robert is boring. (all the time)
So far simple enough, but then there are so many exceptions as to make the rules almost useless. Every beginning student of Spanish is familiar with throwing your hands up in the air, upon finding out that you have to use ESTAR for a location of a building which is certainly permanent and SER for a location of a party which is certainly something that can change. You use SER to tell time which changes continuously. And then there is the ultimate doozy that in Spanish you use ESTAR to say that someone is dead. What can be more permanent than that? — one wonders in bafflement.
Examples of the exceptions:
La biblioteca está a 10 kilómetros de aquí. (ESTAR, even though location of a library is permanent)
The library is 10 kilometers from here.
La fiesta es en casa de Robert. (SER, though the venue of the party can change)
The party is at Robert’s place.
Julio Ceasare está muerto. (ESTAR, even though deadness is a permanent quality)
Julius Ceasar is dead.
Son las 10 de la mañana. (SER, though time is notoriously fleeting)
It’s 10 in the morning.
And then there are many more exceptions. This “to be” dichotomy is certainly one tenacious obstacle to fluency. After all, if you can’t even reliably figure out how to say that something “IS,” then you are in big trouble and fluency would be the last thing on your mind.
Solution: Focus on Essence not on Permanence
Traditionally the student is expected to memorize a list of exceptions (such the use of SER for event locations) but even after studying the exceptions diligently I found that I still made 15-20% mistakes in exercises. The key thing that allowed me to put these difficulties behind was the realization that the criteria for choosing SER and ESTAR that focus on permanence and transience do not constitute a valid model for what goes on in a Spanish speaker’s head. It is perhaps a useful way to introduce the concept, but as a general heuristic is not all that helpful.
The trick is to focus entirely on identity and essence and disregard whether or not the property in question is permanent or likely to change. What does it mean to say that a particular property is essential, that it is part of an identity or essence of a thing?
A good operational definition is as follows:
A property of a thing is essential if it makes the thing that thing and not another.
A useful restatement of that would be:
If you remove an essential property of a thing, it would stop being that thing and would become some other thing.
When a Spanish speaker uses SER it is not necessarily a sign that he or she thinks the property is permanent, but that it is essential, that if it were to change the thing in question would become some other thing. Likewise, the use of ESTAR does not always imply that the quality is temporary, but merely that it is not a part of the thing’s identity, that it would remain that thing even if that property were different.
In scientific terms it is is helpful to think in terms of essence of a system and its state. The essence is what remains the same as the system goes from one state to another.
You can see this very clearly in the way the Spanish phrase “How is X?” is typically translated into English.
Cómo es Robert? (SER)
What is Robert like? (what is his nature/essence?)
Robert es una buena persona.
Robert is a good person.
Cómo está Robert? (ESTAR)
How is Robert doing? (what is his current state?)
Robert está contento por que ayer aprobó el examen.
Robert is happy because he passed the exam yesterday (his current state)
The main difficulty comes from thinking of state as something that is necessarily short. A state could be very long or even permanent. It’s just that it does not determine the essence of the thing and the thing could be in a different state.
Another important way to appreciate the nature of SER is to note the fact that when the object of the verb “to be” is another noun SER is used always and without exception, regardless of the permanence or transience of the nouns in question. This is because when the object of “to be” is another noun the “IS” is certainly the “IS” of identity not of state.
La silla es un objeto para sentarse.
The chair is an object for sitting.
Once we focus on essence and identity as the key for deciding between SER and ESTAR, all the exceptions disappear and the choice becomes natural and automatic. You just need to pay attention to the noun that the adjective is describing and determine whether the property is essential with respect to that noun, not to some larger general context. For instance, as explained in the examples below, the physical location is an essential property of an event but is not an essential property of a person.
1. The physical location of a person is not part of his or her essential identity: Arnold would still be Arnold whether he is Sacramento or Los Angeles.
Arnold está en Sacramento. (ESTAR, location not part of identity)
2. Conversely, the physical location of a party is definitely a part of the party’s identity. If you change the venue from a snazzy hotel to a local McDonald’s, it would be a very different party even if it has the same list of attendees.
La fiesta es en casa de Robert. (SER, location is part of identity of an event)
3. Time and date identifies the present moment. If we say that right now it is 2:45pm on June 23rd, 2008, we’re identifying a particular moment in time. The numbers that the clock shows is part of that moment’s essential identity — if the clock showed other numbers, it would be a different moment, not that one.
Son las 10 de la mañana. (SER, clock reading is part of a moment’s identity)
4. Physical location of a building is an interesting one. If we take the library as a physical building and move it a few blocks over, would it still be the same thing or would it become some other thing? I would vote that it would remain the same thing, and, hence, location is not part of the essential identity of a building; the building would still have the same architectural design, same number of floors, same square-footage. On the other hand reasonable people can disagree: if you moved the Buckingham Palace to the Aleutian Islands, it would probably lose some of its Buckingham-ness. As it happens, some reasonable people do disagree: In Portuguese SER is used for geographical locations and in Spanish ESTAR.
La biblioteca está a 10 kilómetros de aquí.
(I’d be curious to hear a Spanish speaker’s opinion on this. Does it sound wrong to you if one says: La biblioteca es a 10 kilómetros de aquí?)
5. A person’s profession or nationality is generally an important part of a person’s identity. If Robert is a doctor and then he has stop being a doctor (e.g. because he lost his license for some reason) that would probably cause profound changes in his personality. Anyone who has attempted a career change can attest that it is a very transformative experience.
Robert es medico. (SER, profession is part of identity)
Robert es Inglés (SER, nationality is part of identity)
A pedantic reader might object here that for a hypothetical person who changes his job every day, we would still use ES not ESTÁ. That is true, but the key thing is that the meanings of words in a language are a product of compromise of a very large number of people, and, as such, to figure out what “sounds right” to a native speaker, you need to think about how the language is used on average, not in a artificially designed situation. If the majority of Spanish speakers began to change their jobs daily, I would guess they would start using ESTAR to describe their profession within a few years.
An interesting counter-example would be to consider a color of a thing. Today, we would normally use SER: Esta camiseta es verde. (This T-shirt is green) but things will be different in the year 2200 when we have magical T-shirts that change their color to match your mood, based on the blue-tooth signal from you brain implant. When you see your friend Robert walking down the street, you might say “Su camiseta está azul; quizá esté triste.” (His T-shirt is blue; maybe he is sad).
6. Deadness is not a part of a person’s essential identity. This one may seem a bit tricky, but recall that an essential property is one that makes an entity that entity and not some other; something that makes it distinct and allows us to identify it as that particular entity. Once you look it that way, you can see that the fact that Leo Tolstoy is dead does not make him any less Leo Tolstoy. What is essential about him is that he is a great Russian writer, that he is the author of War and Peace and Anna Karenina. The fact that he happens to be dead does not make him any less of what he is. The reason that the use of ESTAR with “dead” is always so confusing is that traditional explanations say that you should use SER with permanent qualities and ESTAR with transient ones, but, as we already noted, that’s misguided — a state of an entity could be permanent; it’s just not a part of its essence.
Tolstoy está muerto. (ESTAR, the fact that Tolstoy is dead is not part of his identity)
Elvis está vivo.
1. Dogfooding is the term used by Google in its official blog regarding the Nexus One phone.
2. Almost all languages follow a word order paradigm. For instance most Western languages use Subject-Verb-Object (SVO) order. The SVO paradigm is the default choice in Russian as well but, as Wikipedia tells us, “Some languages are more complicated: Russian allows all possible combinations SVO, OVS, SOV, OSV, VSO, VOS. Changing the word order influences the nuance of the meaning. Usually the last word in a sentence is emphasized. But other implications are possible. Varying word order is very common in Russian.” True that, confirm I.
This freedom from word-order is achieved through the use of a complex grammar that involves elaborate noun-adjective agreement, six cases, and three genders. Thus, for instance, the adjective “green” can take potentially take 36 different forms in Russian depending on the state of the noun it’s describing (3 genders X 2 numericity X 6 cases). The upshot of this is that the agreeing adjective “feels” its noun no matter of where it is located in a sentence, the case-inflected nouns “know” who is whose object, and the conjugated verbs are grammatically connected to their subjects. The order is thus incorporated into a set of grammatical couplings that are expressed through word endings.
Since the words can be placed in any order, the Russian poet has a lot more combinatorial permutations to work with than a poet from a more ordered language. An interesting indirect illustration of this is that Pushkin, who was principally a poet, is much less known in the West than the big three (Tolstoy, Chekov, and Dostoevsky) and yet for native speakers he is a giant who stands head and shoulders above all others in Russian literature. A text-book definition would say that poetry is a synthesis between rhyme and metaphor. In Russian poetry, rhyme takes a much more prominent place since the greater permutation space gives the poet many more ways to arrange rhyming verses. Translating poetry is always a thankless task, but when it comes to rendering Pushkin, it is nearly impossible since most of the value is locked in the musicality of his verse which itself is enabled by the flexible nature of his medium.
3. Cliffs Notes. Confusing Verbs: To Be, To Know, To Take
4. Technically speaking, the “object” of the verb “to be” is a subjective complement.