If you see nothing wrong with having your ability for your 14th level be "I get a CL2 Magic Missile!" then you've got more wrong with your perspective of mechanics than taste. It quite simply does not compete with alternative choices. Thus, it is significantly unbalanced. Thus, something is wrong. Simple.
IMHO, you don't multiclass for efficiency or to powergame, you multiclass to capture the essence of the PC. That a multiclassing choice doesn't equate to the power gained by continuing to level within a class is totally and utterly irrelevant.
If your PC wants to take his first or second level of Wizard at 14th level and gain MM (no other levels of Wizard gains only a single 1st level spell), there's
nothing wrong with that.
Its not unbalanced- its a natural and realistic consequence of being a magical dilettante.
For me, it strains credulity that 14th level PC to gain a low level of wizardry and gain an ability commensurate with the power of an ability gained at 14th level by a mono-classed PC.
Consider, Michael Jordan, despite being one of the greatest professional basketball players of all time, couldn't hit a baseball well enough to play competitively as a pro. He wasn't even a good
minor leaguer, much less a major leaguer.
(And I'd advise you to tone down things like "then you've got more wrong with your perspective of mechanics than taste.")
ToB: Your caster level is based partly on your character level, you can get and scale abilities from other classes, and you have feat-like qualification for "spells." You suggested all of these things, right after saying you hated them)
Not quite. (And to clarify- I own ToB, read it, and hate ToB's take on martial combat.)
1) Caster level should be
entirely based on caster class levels- different from ToB. I never suggested otherwise. I'll go further in suggesting that arcane caster class levels shouldn't stack with divine (or psionic) ones.
2) I never said anything about getting or scaling abilities for other classes.
3) I didn't say anything about "feat-like qualification for "spells."" I said that spells could have prerequisite spells, like feats have prerequisite feats. Conceivably, some might even require skills.
Kinda like I can't get, say, White Raven Strike if I don't already know at least one White Raven Maneuver?
No, more like you couldn't get White Raven Strike unless you knew a
specific prerequisite maneuver.
For spells, you couldn't learn Deep Slumber without learning Sleep. No other spell would do. Color Spray would be
the prereq for some other spell like Rainbow Pattern. And that same structure and stricture would be for each spell.
To further clarify, what I'm suggesting is that spell acquisition would be more like a tree than ToB's take. Several of the lower levels would be entirely linear- you couldn't take the second level Polymorph spell without the 1st level one, and the 3rd without the 2nd. At higher levels, as the school branches out, you'd still have to have the specific prerequisite spells. Its even possible that certain subschools of a family of spells would be mutually exclusive.