The biggest problem I have with sorcerers and wizards is the number and power of the spells they get. I prefer games with lots of role playing, puzzle solving, and mysteries. Those kind of adventures are impossible when the group has a wizard or sorcerer with divination spells, illusion spells, polymorph spells, etc? Even in combat, a wizard or sorcerer can use augmentation spells to turn into a tank just as good, if not better, than any fighter or barbarian, especially at higher levels. "Dog piling" with low CR critters is all well and good, except the wizard or sorcerer usually has spells that can incapacitate or kill a large group of low CR critters in one shot (unless they can somehow take the wizard or sorcerer by surprise).
The biggest problem I have with clerics and druids is the fact that it makes the characters lazy in combat (sorry, but lazy is the only word I can think of that fits). Why worry about strategy and tactics when you can just stand there and slug away, the cleric or druid healing your character whenever he or she gets low on hp. Even things like ability drain and level loss becomes trivial when the cleric or druid can just instantly repair it.
How about I make it so that no character can have more than 10 spellcaster levels total (including bard, paladin, and ranger levels where they gets spells, and prestige class levels that get spells). I could also ban certain spells (maybe even entire schools), and maybe reduce the number of spells major spellcasters get. The clerics ability to spontaneously cast cure spells has to go (maybe give clerics a bonus feat or two instead).