There's no disable inducing caster items like in DotA so you are pretty much shafted if you're playing a caster without an inherent disable late game.
That's my only real problem with casters, item selection. Mana/regeneration items aren't choices, they're necessary. The only choice after that is if you want HP to not die or AP to not suck. AP item selection needs a boost since a lot of heroes can't really get much use out of rylai's, lich bane, abyssal scepter, so they're stuck with rod of ages, zhonyas and archangels.
Need more pure AP items for damage casters IMHO. Mejals is great but having more than one isn't viable.
Activated disables like DotA has would be a step towards making casters with no disables better picks, but... It'd also just make casters with disables better at disabling, and CC is way too prevalent already.
Also, casters being dominant early game is not a big deal in LoL at all, IMHO. You still need physical DPS to push towers, and your enemies' carries aren't going to be very hampered by your early game ganks because they don't lose gold for it and aren't out of their lane as long as in DotA. Not to mention, most carries (Yi, Jax, Ashe come to mind), have great skills for ganking as well.