Class by class, here is how I see it:
Barbarian: none [1]
Cleric: restrictions based on religion only
Druid: as per cleric
Fighter: none
Monk: none [2]
Paladin: Lawful only [1]
Ranger: none [1]
Rogue: none
Socerer: none [3]
Wizard: none [3]
[1] Paladins, Rangers and BArbarians are all variations of a theme. They are three approiaches to fighting men. The way the fighter is currently defined (as a feat-gathering machine) should be set up so that Paladins and Rangers are simply Fighters with certain fairly common feat chains. Otherwise, the true Paladin and the true Barbarian should be handled as prestige classes. In my opinion. If I were writing this. But I am sure this is far too radical.
[2] Monks belong in Eastern games, the monks of the west this does not fit and should not be included in the core D&D book, which deals primarilly with Western myth. Again, in my opinion. If I were writing this. But I am sure this is far too radical as well.
[3] Sorcerer and Wizard. The main problem I have here is the fact that the game deals with these as seperate classes. Same spell listing, different rules. This was (in my opinion) a mistake. They should be the same class -- but set up as "if you want your campaign to work this way, then the Sorcerer does this; if you want them top work this way then they do this" -- such that a campaign has either one or the other but not both (by default; a DM is free to do as they choose). Again, in my opinion. If I were writing this. But I am sure this is far too radical as well.