You're thinking in the right direction. Warlocks are far below Rangers and Rogues in damage, and they make up for it by getting more cool tricks. One of their cool tricks is using a magic item to bypass a tactical restriction ("must curse closest").
He's paying the same price that a Rogue would pay for a Frost Dagger. This is interesting, because a Frost Dagger, combined with Lasting Frost + Wintertouched, allows a Rogue to bypass several tactical restrictions -- it basically gives him free Combat Advantage next round if he hits a foe this round.
So yeah: it's powerful, it's cool, and that's okay.
Cheers, -- N
Again, I don't think your comparisons are anywhere near equal. For your rogue example the rogue would obviously be hitting (ie using an actual roll) on a bad guy. And for him to get that Combat Advantage it'd have to be a non-minion. But either way, he is rolling to attack, and so therefore has a chance to miss. The warlock on the other hand has no range limit, and doesn't have to roll, and can take down minions.
I can see that we aren't going to agree, and so I wonder if other people are more in your camp or mine, or just haven't noticed this yet. I wonder?
cheers,
Tellerve