I like the idea of stealing a creature's power and using it directly - that's the fun part of the class. But, it could be done in a better way - you use an encounter, and the attack and damage code is predetermined... you're simply stealing the tack-on effect.
There's some problems with the old way of doing stuff.
1/ Monster powers no longer have a level. This is huge. Monster spells and spell-like abilities always had a
direct comparability to PC spell level -- those that didn't just replicate PC spells had their level stated explicitly.
This means you can't really tell which powers are appropriate to steal. The 3.5e dude had a level cap on his spelltheft for a reason, I'd assume.
2/ In 4e, monster attacks tend to cluster by "theme", since PCs can't just pile on buffs even when they know what to expect. Thus, a fire giant's attacks will do extra fire damage, and he'll resist fire damage, and this is okay. However, it means stealing an attack power will often suck -- no more blasting an Ogre Mage with his own Cone of Cold; instead, you're more likely to find undead throwing necrotic damage at you, which won't really hurt if you throw it back at them.
3/ Recharge. They're neither at-will nor encounter; and worse, there are varying degrees of rechargeable, which imply some powers are stronger for their monster's level even within the group "recharge", which is assumed to be somewhere between at-will and encounter in strength.
So yeah. I don't think the system supports "balanced" theft in the way 3.5e did.
Cheers, -- N