Is that they are a caster with no magic. Or at least, not enough to last very long and therefore they have to be very conservative with their SP, especially on longer quests.
Of course wizards need a down side compared to the sorc's, and that downside is half the SP. But that's quite harsh IMO. It makes them stand around doing very little in most battles because of their constant need to conserve SP. Having a caster with no magic sucks.
A variety and a selection of spells does you no good if you lack the SP to cast them.
My recommendation for DDO: To hell with the rules. They weren't designed for an MMO. Let wizards at least have a better BAB (or whatever its called) with weapons so that in the common instance that they have no magic, or are trying to conserve their tiny SP pool, they dont stand around doing nothing.
All this is my opinion. Feel free to flame me, but you know i'm right. No other class is forced to stand around doing nothing.
Their tiny SP pool is such a disadvantage, only when DDO implements about twice as many spells as there are now would having a wizard be worth it, so sorcs would have to make serious and difficult choices, rather than laughable choices ("hmmm..do i get haste or fog cloud? such a dilemna...").