I think the Hybrid Class playtest rules are a good start, though it has rough spots. I think it is works well for the hybrid cleric and warlord, because you get a healing encounter power and one of their good at-wills (thinking righteous brand and commander's strike), and obviously access to all the powers. I think it works well for the wizard, because the cantrips are fun and flavorful, if not powerful, and the main attraction of a hybrid wizard is access to all the spells.
The strikers and defenders are where things seem to break down. You don't want a hybrid striker/X to be as good as a pure striker, so you somehow have to limit the ease with which you access the striker's bonus damage. They have addressed this by making the bonus damage (Sneak Attack, Hunter's Quarry, Warlock's Curse) only accessible when using powers from the corresponding class. But the restriction does seem a bit artificial. I am ambivalent on this implementation -- I could see it working in either direction.
The fighter, paladin, and swordmage are another story. I think the hybrid fighter's Combat Challenge should work on all attacks (or maybe all melee and close attacks). The fighter's defender shtick comes from Combat Challenge and Combat Superiority -- Combat Challenge is more important (hence its inclusion in the hybrid fighter). I think the hybrid fighter's Combat Challenge could use some sort of limitation, but the fighter power limitation is not to my liking.
The paladin's defender shtick is a weird combination of Divine Challenge and its ability to heal and bolster his comrades through Lay on Hands and other powers. I think Divine Challenge <<< Combat Challenge in terms of how integral it is to the paladin's defender role, so the hybrid paladin should probably have Divine Challenge as it is written in the PHB, no changes.
The swordmage is trickier -- Swordmage Aegis is its sole defender trick, outside of any specific effects granted by powers. So you can't give a hybrid swordmage that, otherwise the hybrid swordmage begins to equal a full swordmage in its defender prowess. So I agree with the need of a decrease, but perhaps not their specific implementation.
One thing I didn't mention with defenders is AC. Since AC is especially important to them, since they have to put themselves in front and in harm's way, the loss of armor proficiencies (depending on how you hybrid) is particularly devastating. It can be mitigated somewhat by taking a complementary class where you are boosting Int or Dex, but if you don't...maybe some sort of intermediate solution is needed re: armor proficiencies. For the swordmage in particular, I'd probably implement some sort of reduced Swordmage Warding, like +1 or +2 AC if you have a free hand, otherwise no bonus.
The PHB2 classes and how they would interact with the hybrid classes must be considered, so here are my thoughts on each class and how they might work (or not work):
Avenger: Oath of Enmity would only work with avenger and avenger PP powers. Probably would have to stick to that, if only because rolling two dice and its interaction with the accuracy of other class powers could have far-reaching effects.
Barbarian: Rage Strike could be limited to once per day. But the limitations on daily powers for a hybrid class is already limiting Rage Strike indirectly. In truth, the barbarian's striker firepower is not part of a class feature, but is rather locked up in the class powers -- and those are fully accessible to a hybrid barbarian. Could be the hardest class to implement as a hybrid for this reason.
Bard: I think 1/encounter Majestic Word is pretty obvious. And since you are already a hybrid class, and can still take one multiclass, you've largely replicated the bard's multiclassing insanity (at least the nonextreme forms).
Druid: Wild Shape as is, and a bonus beast form at-will. But then what is the hybrid druid giving up? Another tricky one. Maybe Wild Shape as a twice per encounter ability?
Invoker: You probably can give the hybrid invoker one part of the Covenant feature. The question is: which one? I would lean towards the Channel Divinity power, but I can see the covenant manifestation as well. The tricky part is wording this so you don't give the hybrid invoker the full covenant ability, so they don't access the special benefits in the related powers, since that seems to be something that should stay with the full class.
Shaman: I think the full Call Spirit Companion and 1/encounter Healing Spirit makes sense. How important are the spirit's OA abilities? I am not sure -- maybe they could be included as encounter abilities, or excluded all together.
Sorcerer: The extra damage feature is the obvious one here. Is keeping it limited to arcane powers (as opposed to sorcerer and sorcerer PP powers) ok?
Warden: Nature's Wrath, plus the powers warden's fury and warden's grasp, represent the core of the warden's defender prowess. I think it makes sense to limit Nature's Wrath to marking one or a few adjacent creatures and/or making the hybrid warden choose one of warden's fury and warden's grasp to have (not both).