I see only three problems with the Warlock.
1. The fey pact warlock is lousy at low levels. It gets better by paragon tier, but at low levels its damage is boring. 1d6+cha +1d6 is functionally the same as 2d6+stat, ie, the same as fighter with a hammer. And its pact boon is difficult to use at low levels because being able to teleport outside of your turn when an unpredictable event randomly takes place is not that strong of a benefit. At higher levels you can get feats and items to store your pact boon for later use, to share your pact boon with allies, and you can even get a second pact boon for when you don't want to teleport anyone. But at early levels they have low damage and a rarely useful pact boon.
2. Star pact warlocks are obnoxious. If you go charisma/intelligence, you have half as many pact powers to choose from as compared to a warlock of a different pact, and even worse you can't use your pact at will. So that's right out. If you go constitution/intelligence, you can't access as many pact powers as another pact. If you go constitution/charisma, you end up with AC problems that have to be fixed with a feat and points in strength, and you end up having lousy pact bonuses on your encounter powers because you can't afford much intelligence, meaning that even though you can use a wider variety of pact powers, they suck.
3. Requiring that all warlocks take Eldritch Blast was dumb. What they should have done was require all warlocks to take their pact at will, and then to take at least one at will that is not specific to any particular path. This would make Eldritch Strike usable, and would allow them to design more warlock at wills without them only being available to humans and multiclassed characters.