In order to allow players into a world, one needs to determine allowable classes for the characters. Often, all the standard classes work, but let's think through them.
Barbarians are an iffy one for me. I don't like to allow characters to be barbarians without a good reason for ending up in a campaign group. I'm certainly not allowing players to throw on a level of barbarian without some extraordinary circumstances. I don't think, though, that I will necessarily throw the barbarian class out of the campaign. A berserker is a feasible thing, I think.
Bards are always in. After all, what better class for most adventuring groups than the one who would like to retell the stories? I may tweak the way they weave magic just a bit, though, but it'll mostly be flavor, rather than substance.
Clerics will, of course, have to follow a deity, and the deity will determine some things, such as allowable weapons/armor. Not all clerics will be able to wear every sort of armor, at least if they want to please their god.
Druids will have to follow the twin gods of nature, though some may favor one very strongly, even to the point of denying any allegiance to the other. Those druids will use only the powers that their choice of god would grant.
Fighters won't change, of course. Why would they?
Monks are out. I don't like them in a setting like this, since the Eastern flavor just doesn't fit the Western civilization. If one of the kingdoms gets that flavor, I may reconsider, but they are out for now.
Paladins will follow a code of honor, and will be allowed in both chaotic good and lawful good flavors. I've always liked the idea of a paladin that would do good, but not give a damn what the law allowed, as long as what he was doing was for the greater good. However, the chaotic good paladin still follows his own code of ethics, and will be even harder on himself than the LG flavor, since he doesn't answer to anyone but himself.
Rangers, as I mentioned in the Pantheon section, will be forced to choose a path, gaining spells fitting either the path of growth and life or the path of power and destruction. I'll have to rework their spell lists a bit, but I think it'll make rangers more interesting.
Rogues are always okay. I can't imagine a setting where a thief wouldn't fit.
Sorcerers will be perfectly okay as a starting class, but there will be special story requirements to explain someone suddenly gaining their first level of sorcerer otherwise. "Hey, I suddenly noticed that I have natural magic!" just doesn't cut it for me.
Wizards, too, will be in. There will be wizards researching new spells, others teaching a few students, and still others going out looking for the lost spells of other civilizations. Spells will be a valuable commodity, not easily handed out. If a PC wants to get a new spell, s/he will be forced to provide a spell of equal value or do something to earn it. But there will be plenty of spell trading, and an entrepreneurial wizard will be able to not only acquire spells, but even make a profit while doing so.
As for some of the common prestige classes, I'm taking out the dragon disciple with my description of sorcerers Other classes will be in if they fit, out if they don't. Obviously, any classes that require monk levels are out, as are racial classes that require races I'm not allowing in. The rest will be determined on a campaign-by-campaign basis.