Dark elf thieves are fun, but very tough to play while not getting ripped apart to pieces quickly. Here are some ideas to make them more dangerous, while not losing the frailty that is all part of the Elf Experience:

"Exploit Vulnerabilities"
The drow thief is wily and cunning, and knows more than most about his enemies, and can hit them harder when wielding a weapon their opponent is vulnerable to.

"Focused Faerie"
On a knocked out opponent, a drow thief can target his inherent ability to cast pink auras onto an item in the opponent's hands or inventory, making it strike much more weakly than normal.

"Arcane Fire"
Ready to start! Can recite offensive scrolls in battle. (I don't know that this is as awesome as it sounds, there is scroll lag!)

"Sneaky Stealer"
Elf thieves' victims will say, instead of yell when failing to steal. Mobs have chance to not attack.

I'm only really a fan of the "focused faerie".

I don't like the idea of giving them an assassin edge (exploit vuln).

The scrolls thing is kind of strong, imho. Having said that, if I was doing drow thief, I would me making pretty heavy use of offensive scrolls at hero.

Hah, didn't even know about the assassin edge.

Scrolls in battle should really be high-cost an mystic skill, I think, with high int best leveraging