Okay, so we want to use a unified caster level progression, but have enough distinctions within each class that it affects how you cast spells. Everyone knows two spells per level in a casting class, and they can spend 10 minutes performing a ritual to cast another spell from their list, at the cost of expending a slot of the appropriate level.
Sorcerers, for instance, wouldn't get the 'Ritual Casting' ability, but as they advanced they could maybe get bonus spell slots, which could only be used for sorcerer spells.
Wizards, by contrast, could pull off ritual casting the fastest. He just needs a spell-book and the necessary power components, and he can cast a spell out of his book in two rounds (as a rough example; playtesting could change this). Normal ritual casting takes 10 minutes. In addition, the wizard has spells known that he can cast at will (just like the 3.5 sorcerer can), but he also has the option to prepare spells out of a spell-book.
Clerics get bonus spell slots which they can only use for their domain spells.
Druids don't get any special rules for casting. Bards need to have their spell list switched to the 0-9 spell levels everyone uses, and they get only 1 spell per level.
Classes without spellcasting . . . hm.
I realize the problem is that if you're a Ftr 19/Wiz 1, sure, you only know 2 spells, but you've got all the spell slots of a 20th level caster. Now, if you pick 2 9th level spells, you can only cast, like, 4 or 5 of those. But if you pick, say, magic missile and fireball, you've got a ridiculous amount of evocation power. If you'd grabbed maximize spell, you can still be fairly impressive.
So here's the easy solution. Normal classes just provide 1/2 normal caster level progression, and you obviously don't get any spells known when you take a level in a non-casting class. So the Ftr 19/Wiz 1 has a caster level of 10. Yes, he can know any two spells of levels 0 through 5, and yes, he can cast them a lot. But that's really not overpowering at 20th level, is it?
I still need to figure out how to keep Ftr 19/Wiz 1 from abusing rituals, and how to handle half-caster classes, but I think this is a start. What do you think?