I think the difference between ser and estar has not been asked yet.
I know the general rule is that ser applies to permanent states, but there are some tricky cases for non-natives, such as:
I cannot come up with a new general rule that will give you the right answer in every case. Ser for permanent sates, estar for temporary ones, or perhaps better, as André Lemos puts it in his answer, ser for essential features and estar for accidental ones, is the best I am aware of. But I can clarify your examples. Note that I from Portugal, and while I’m not aware of differences between Portugal and Brazil is this respect I would be surprised if there weren’t any at all. Note too that in some of my examples the other verb might be acceptable in some circumstances, but if you stick to the rules below no competent native speaker will tell you off. I hope this helps.
Location – Basics Use ser for things that are intrinsically immobile. So a cozinha é aqui (you've got that one wrong), unless you're talking of a portable kitchen. Onde é a festa? because a festa takes place in one place. The festa itself is temporary, but while it lasts it is in one place. So the same applies to a reunião or a jantar or events in general. For events ser is the only option, but for other things you can substitute ficar for ser: onde fica a cozinha? Manaus fica na Amazónia.
Location – within sight. Suppose you’re looking round a square for a small shop you’ve been told is there, and you can’t find it, and are becoming impatient. It will be natural to say onde está a [optional expletive] loja? as if you were looking for a cat hidden in your living room. And someone might reply, está mesmo atrás de ti! In general you can use estar to indicate something within sight. Ser and ficar can also be used in these situations, but estar feels almost the most compelling of the three.
Location – surroundings. A comment below gives the example a casa está/é no meio dum bairro problemático. In this case está ou fica sounds definitely more natural than é to me. The ex-post rationalisation I can come up with is that you are not giving the location, with is an essential feature of the house, but only the sort of surroundings the house is in, which could change in theory – the neighbourhood could become less problematic. It will be the same if you say a casa está no meio de um bosque. If there were only one bosque, then it could indicate the location, and it would be natural to say a casa é no meio do bosque; segue por esta estrada e vais encontrá-la facilmente to indicate location, or to say a casa está no meio do bosque; acho que me cansava de tanta tranquilidade to emphasise the sort of surroundings. Fica is more versatile and fits well in both cases.
Location – here your are. Aqui está or cá está is an idiomatic expression used to show someone something. So I could drive you to the shore and say, with a sweep of my arm, aqui está o Oceano Atlântico, or, when showing you around the house, aqui está a cozinha (we use this especially for things of some significance, or when you’re eager to see the thing, so it would have to be some kitchen). We could equally say aqui tens a cozinha.
Health states/diseases. Use ser for chronical conditions, estar for diseases you expect to go away. So ele é diabético, because if you have diabetes you typically have it for life. Same with ele é asmático or ele é esquizofrénico. But ele está gripado because you expect the gripe to go away after a few days or weeks. In fact for most diseases you don't have an adjective and will say ele está com/tem febre/sarampo/pneumonia.
Occupations/hobbies. Never use estar. If you do not expect someone to keep some activity for long enough to merit the use of ser, you can say ele trabalha como consultor or ele está trabalhando/a trabalhar como consutor or, to give yet a stronger sense on transiency, ele anda fazendo trabalho de consultadoria. But never ele está consultor/escultor/futobolista. If he is a consultor then ele é um consultor even if he takes a break from it.
Young/old. You use ser as you know. I suppose that’s because that does not change overnight, and does not change at one’s will; you just age according to the laws of nature. You think of someone's age range as an essential, not accidental, feature of that person. Note that for inanimate objects we can use estar. O carro está novo if it is in top-notch condition despite its age. Same for a house or a shirt. You can also say of a person ele está muito velho to mean that he looks unexpectedly old given, say, his actual age or his appearance last time you had seen him.
Dead/alive. You actually use estar mostly around the time of death or when someone might be dying, so when the state may be changing. Say, you arrive at an accident and ask ele está vivo? or ele está morto. If someone has been dead for some time, it is more natural, for me anyway, to say ele (já) morreu há seis meses. And you say ele ainda é vivo to mean that he is still living despite old age, but you say ele ainda está vivo to mean that he is still alive despite fear of imminent death, as in an accident.
Friendship. That’s perhaps the trickiest one. Maybe it is just out of an optimistic view of human nature: If elas são amigas they’ll be friends for life. We can say elas andam muito amigas to convey the idea that their friendship is new or unexpected or expected to be short-lived.
The difference between ser e estar is very well defined, in that it presents no difficulty for native speakers. It could be argued that this is the product of historical accommodation, that this is a complicated and manifolded distinction that native speakers become apt at dealing with, as they do for instance with verbal and gender inflections, but it is not the case.
What happens here is that the Portuguese language expresses the aristotelian distinction between essence and accident, that has also become natural in many other romance languages (French being a notable exception, there may be others).
Etimologically, what we have is that the verb ser comes from the latin [esse], and also from [sedere] (to be seated) whose sense was weakened until it was united with [esse] on a semantical level, but overcoming it morphologically, and that is why we have ser, both in Spanish and in Portuguese. Estar, on the other hand, comes from [stare] (to stand), which was later associated with accidental qualities in general.
The problem is that this distinction, conceptual as it is, will often become "encrypted" in different ways, in a multiplicity of use cases, many of which are quite subtle and difficult to understand for a non-native speaker, because these cases do have a historical component that hides the conceptual distinction in ways that are hard to predict and, unfortunately, have to be memorized.
A contrasting example: you may use estar, because you want the verb to express an accidental feature of the subject:
Ele ainda está novo.
In another situation, you may want to circumstantially attach the object, but as an essence, to the subject. You will then use ser:
Ele ainda é novo.
All the examples in the (very interesting) other answers to this question illustrate the different strategies that we use to deal with this genuinely philosophical linguistic problem.
I find it very difficult to set rules for the use of "ser" and "estar". Jacinto has done a good job, though, and presented a detailed answer which certainly helps a lot. In ptBR, however, I see more exceptions than any rule could afford.
In ptBR, the following are permanent conditions but, depending on the situation, "estar" can be used.
Not permanent conditions, but we can use "ser".
Elé ainda é novo.
Pode comer que o peixe é fresco.
Vai ser uma tarde chuvosa, mas depois o tempo abre.
I'd say there are basic rules as Jacinto pointed out but there are set phrases, special uses, and transatlantic differences, so one has to be aware of them.
I'd compare the use of "ser/estar" to the use of "do/make" in English. The foreign student is told that "make" refers to the product of an action (an idea of construction - "make a cake") whereas "do" refers to the process rather than the product, but there is much more to it. Idioms and set phrases abound (make a difference, make an effort, do your homework, do a favour, do your hair, are you doing history at school?, You have to arrive at 5 p.m.? You'll never make it.)
