I think people are selling Unnatural Vitality way too short. Consider what happens every time you drop below 0 HP- you go prone and you drop what you're holding. Some class abilities might well turn off too, like Swordmage Aegis. Even if the healer smacks you immediately thereafter with a healing effect, you still have to spend a move action standing up and probably a minor to pick up your gear- assuming your enemies haven't snatched it up away from you while you're lying there bleeding out.
Revenants don't have to deal with that. Give them a bandolier of cheap healing potions, and the only possible way to put them down is to either take them from positive to -half HP before they get a chance to act or run them completely out of healing surges. Let them take the "use a minor as well as a standard while Unnaturally Vital" feat, let them grab a minor-action source of healing, and getting pounded into the negatives won't even break their stride. Moreover, enemies will keep pounding on the revenant because he's still standing up, and every point of damage they do to him in the negatives will be wasted the minute he gets a surge.
Combine it with their paragon path's "Unkillable" level 16 ability, and every revenant gets an absolutely obscene amount of effective temporary hit points, because it doesn't matter how deep they are in the negatives when they get a healing surge. Instead of worrying about healing them when they get bloodied, you start to worry about healing them when they _go negative_... and the revenant operates at 100% until the minute they keel over dead.
Personally, I really like the feel of this race. Unnatural Vitality turns them into high-quality zombie unstoppables, the sort you have to hack to pieces before they stop coming at you. With that in mind, I think Con/Dex is a perfect fit for the "living corpse animated with the awfulest livelieness" thing they've got going.