Being a specialist wizard at the cost of giving up two schools of magic is possibly the single most important class feature of the wizard, and we can't do it in DDO.
Well, this is how they should do it:
The specialist wizard chooses two schools of magic, he cannot cast spells from these schools. This includes scrolls, wands, or memorization. In return the specialist wizard gains a second spell point pool reserved for spells from the specialist school. The primary spell point pool stays the same size, the specialist pool serves as the "bonus spell slot" for each level that a specialist wizard gains for specializing. As the wizard casts spells from that school, the dedicated pool subtracts their cost until exhausted. Once gone, the costs for all spells are taken from the primary spell point pool. All spells not from the specialized school are always taken from the primary pool.
Due to our extremely limited spell selection, outlined by barabel below, we should probably only have to ban one school of magic instead of two.
Simple.