I like Prestige Classes when they propose something new , both from RP point of view and new abilities or specific powerful focuses.
What I like much less are PrCl that pretend to be a good choice while they effectively simply merge two base classes, both in concept and abilities: there is a very good Multiclassing system to do this, and a PrClass that simply makes you half cleric and half warrior is not needed at all (unless you're simply looking for a way to skip multiclassing penalties).
Unfortunately there are some of the WotC PrCls that have really few new abilities (although sometimes disguised under new names).
I like balanced PrCls, although I don't necessarily complain if a PrCl which can be taken only at high levels is effectively more powerful than normal (but Book of Vile Darkness prestige classes go really too far

).
I like PrCls with lots of requirements. I appreciate when there are also RP requirements, if that is in line with the PrCl concept, but also I think it's important to have "numbers" as requirements. After all, what is really in common among ALL the members of a PrCl? The 1st level abilities and all the required abilities! ALL Loremasters cast at least 3rd level spells, know a lot of divinations, and have good Knowledge scores. A bad example is Contemplative: all comtemplatives have only 1 thing in common, 10 ranks in Knowledge(Religion), which basically EVERY Cleric has aften 7th level.
And yes, punitive requirements with nothing to do with the concept are really a low point against the designer (why should all Blackguards be usually sundering?)
I also appreciate race/gender requirements; it lets you feel proud you're an Elf if Elves only teach their own kind about the secrets of Arcane Archery.
One more comment:
After all, why should someone with 10 Con and the Endurance feat get in when someone with 18 or 20 Con couldn't? Why should someone with 8 Dex and 10 ranks in Hide (overall +9) get in when someone with 18 Dex and 6 ranks (overall +10) not? I understand why it is done in terms of restricting entry to characters of a particular level, but I chafe at the meta-game restriction this imposes.
Although I agree with you that the Assassin guild has little chance to know if your +10 in Hide comes exactly from high ranks or high Dex (well... characters don't really know that they have "ranks" or "abilities scores" after all... they simply notice the consequences, that is the average result of their own checks), the requirement is put in RANKS not only to set a minimum level, but also because ranks never go down (except when you're level drained, but you should lose the last levels you have gained). Instead, you Dex can go up/down for several reasons, permanently or temporarily. What if you are always wearing an amulet that gives you +10 Dex, and have 5 ranks in Hide; do you think you are the same as having 10 ranks? It's definitely a playability issue.
