Tier 1: Common (Found in nearly all D&D places and settings)
Fighter
Rogue
Cleric
Wizard
Tier 2: Uncommon (Hard variants, complex hybrids, and classes with their own system)
Bard (bard songs can't be fit into a feat)
Ranger (either a class or an ugly multiclass)
Paladin (see ranger)
Druid (wildshape is too broken as a feat. trust me)
Sorcerer (literally the definition of rewriting the wizard as a non-bookish internal caster)
Warlock (uses its own magic system)
Monk (see ranger)
Tier 3: Rare (either not common in the setting or a soft variant but has enough legacy and class features to be a class of 3+ themes)
Barbarian (could be a class or several themes one for each type: martial rager, primal rager, anti-mage...)
Assassin (really only has 3 features:: assassination, poison use, and minor magic. Can be a base class with all three or 3 themes)
Warlord (could be a class or several themes one for each type: inspiring, tactical, bravura)
Psion (variant wizard)
Cavalier (nondivine paladin perfect of a reputation and honor game)
Shaman (variant druid/cleric/wizard, could be a few spirit based themes or a caster with its own spell list)
Tier 4: Theme (classes that don't have enough unique features to really be a base class)
Seeker
Invoker
Ardent
Warden
Battlemind/Psy Warrior
Artificer
Swordmage
Specialist Caster
Specialist Warrior