1) Let me first state that I love D20.
2) I prefer
HERO and
Mutants & Masterminds. Why? Infinite flexibility.
The closest D&D came to this was in the 2Ed Player's Option stuff. When 3Ed was announced, I hoped they'd use Player's Option as a launch point. They didn't.
Personally, PrCls are both curbs and enhancers to creativity. Like someone else said, if you just assembled the feats, skils, powers, etc., and let people build their characters out of those, you'd be able to build any PC concept you want without PrCl's. But WoTC didn't, so you can't.
PrCl's at least open up possibilities, especially to players who don't have the time to learn the intricacies of the system to the point of warping reality...just like Kits did. (There, I said it!)
However, by the very nature of D20, they still restrict the options of the truly creative.
FreeTheSlaves
If a player can't make a cool character concept out of a human & a core class, adding prc is merely dressing up the waste.
That statement is offensive and erroneous. Were you to examine my PC catalog, especially those from other systems, you'd see you were in error. But I'll just go back to 2Ed's Player's Option era.
I had a character concept: a Cleric/Warrior type based on heroes from Finno/Russian Mythology. Those guys had many interesting magical powers, but they didn't deal with undead much. I used Players Option to create him. My PC was able to cast MANY Spheres of spells-and even Wizard Abjuration Spells- in full armor, but only 3 on his TOTAL list (projected out over 20 levels) were offensive- he was to buff his allies and help the downtrodden, not destroy his enemies. Despite being a cleric, he couldn't turn undead, but could inspire rage. He fought almost as well as a fighter with his scale mail, Longbow, Great-Axe and Maul, and because of his tribal background, could track as well as a Ranger of equivalent level. His singing skill was impressive and inspirational, but not magical.
At first level.
I cannot make that PC in 3Ed using core classes alone-perhaps not at all. Wizard levels, even as a specialist, would add offensive spells that no Finno/Russian hero ever cast, and the Bard list completely misses the point of this character (except for the singing). The Scale Mail would impede his arcane spellcasting. No core rule exists for substituting Turn Undead with other abilities, and the current Domain system eliminated at least half of the clerical spells that fit the concept, while adding many that didn't. Even the Gestalt rules would result in an overpowered version of this character concept.
Then there was my 1st level Albino Minotaur Ftr/Mage (Tu-Maa Ghostwalker) based on Native American mythology and culture...
In no way is it possible to play an armored spellcaster who is at home in his shell while casting spells wtihout resorting to rules outside of the norm.