I really dig the basic, three-class scheme of D&D. They are broadly defined, broadly capable, and free to develop in any direction the player should choose. I'm a little torn on the thief. I like the basic idea that there is a need for a stealthy and/or mechanically competent character. I like the idea that said character is shady, with amorphous morals. However, once you give a class the ability to do something, then, by implication, you've denied the ability of the other classes to do it. The most cursory of examinations bears this out.
I am not a fan of subclasses, nor of the plethora of classes that have appeared in supplements and periodicals. I feel that the foundation upon which they are built is too narrow to allow them to function in the adventuring party, without compromising what should be the core tenets of the class. To wit:
With the stricture of personal alignment, plus the limitations of associating with others of like mind, this alone is enough to make the paladin a questionable class to allow. Also, as Lawful beings, they should have serious problems with killing in the name of acquisition. They have a vow of poverty hanging over their heads, so why would they go on a treasure hunt in the first place? Unless the campaign is designed to be some sort of holy quest, paladins simply do not fit with long-term play.
This one is fairly simple. Why would a wilderness warrior, sworn to defend a certain area, seek his fame and wealth in a dungeon? Pursuing or tracking fell creatures to an underground lair, sure. A serious career as a glory-and-riches dungeon delver, hell no.
See Ranger, multiply by 10.
Head east, Caine, there's no place for you here.
Killer-for-hire on an extended career branch as a dungeon delver? I don't see it. Purpose-hired by the party to take out a specific threat, absolutely, but nobody in the party should keep an assassin on retainer.
Obviously, there are plenty more, but I think you get the idea. I have no problem with these as NPCs. I think they are more than fine when they are in the game for a specific purpose related to their raison d'etre.
I also feel that, in most cases, the subclass actually outperforms the base class. Paladins and Rangers have all the benefits of the Fighting-Man, plus all their cool class abilities. The assassin fights much better than the thief, has thief skills (albeit at a delayed development), and trumps the Backstab with the auto-kill Assassination Table. Even if that fails, he still does Backstab damage.
No, for my money, the Big Three are the Only Three. The rest are too purpose-driven, but make fine additions as NPC henchmen or villains.