What are the one or two overpowered combinations in your opinion? I ask only because I have gone out of my way to avoid unpublished preview content like hybrid classes. When DM (which is normally) I tend to allow 98% of all WotC published content ... but only after I've digested the books contents or its been put into CB and/or gone through some 'peer review.'
Some examples: Rogue|Ranger or Ranger|Fighter, focused on choosing minor action or immediate action encounter power attacks. The main limitation of many of the hybrid abilities is that they can only be used with powers of that class. This is a very elegant approach - typically, a Rogue|Ranger could thus use rogue powers when he has combat advantage (and gain sneak attack damage), and then switch to ranger powers when he does not (and gain hunter's quarry damage). Similarly, a Ranger|Fighter can use a fighter power and mark his enemy when he wants to serve as a defender, and then switch to a ranger power and use hunter's quarry when he wants to be a striker.
The problem is that minor action attacks throw off that balance - the Rogue|Ranger can use a standard action Rogue attack and use Sneak Attack, and then a minor action Ranger attack and use Hunter's Quarry. So double the striker damage in one round - he might run out of such powers after 3-4 rounds, but has had a significant damage boost until then. Or the Ranger|Fighter who keeps up striker-level damage while still able to mark his enemies.
Of course, none of this is the end of the world. Getting Hunter's Quarry on top of Sneak Attack translates to an extra 1d8-3d8 damage - there are plenty of non-Hybrid builds that can get similar damage bonuses in a variety of ways. It is only really so powerful because the strategy of spamming minor action or immediate action attacks, in addition to your normal standard action attacks, was
already pretty high up on the damage meter.
Similarly, there are plenty of other ways to add marking ability to a non-defender, if one really wants to. The hybrid Fighter|Ranger isn't breaking the game. But it does come across as a bit overpowered to fully get the best of both worlds, and an oversight in the design for the way hybrid abilities work.
I think the Hybrid rules are certainly still worth allowing - they allow a lot of fun, flavorful builds, and in the end, an optimizer can design something overpowered with or without them.