Why imperfect subjunctive? Quisiera
In making polite requests, I understand why we might use conditional tense: Querría, me gustaría, tomaría, and so on. I have read that Quisiera is also used this way, but I don't understand it. Quisiera is Querer in the imperfect subjunctive. How does that make sense as the primary verb of a sentence? Doesn't this communicate a meaning (to a waiter at a restaurant, for example) that "I would have wanted the glass of water if you weren't so slow in getting here?" 
3 Answers
I remember Lazarus expalaining this once or twice:
This is an exception, and there are historical reasons for it, since the ending -ra wasn't originally a subjunctive tense, but an indicative one (in Latin).
This conditional/courtesy usage is only possible with the -ra endings (never with -se), and only for "querer", "deber" and "poder". The result is exactly the same as "querría", "podría" and "debería", in conditional, as you correctly guess the first time.
"Querer" is used all the time by everyone; "deber" is used sometimes, but "poder" is more rare, and almost not used in certain regions.
Querría pedirle un favor - Quisiera pedirle un favor Deberías tener más cuidado - Debieras tener más cuidado Podría ser posible - Pudiera ser posible (!)
They are all used like the conditional for advice and courtesy.
Here is something from Izanoni:
I think you are correct pesta. Many people say quisiera when they should say querría, in other words, they are using the imperfect subjunctive when they should be using the conditional. So many people use quisiera incorrectly that it's assumed to be correct. However, this is only what I learned from my Colombian tutor as she slapped my ears, I could have misunderstood 
In Ecuador, the imperfect subjunctive entirely replaces the conditional for querer, that is the conditional tense is never used and this is emphasized in textbooks.
