Resistance halves the damage, while vulnerability doubles it. Simple, elegant, scales well.
Edit: and no surprise rounds, just a -20 to initiative if you're surprised. Different... will have to see how that plays out.
Yeah, like with 4e, there's a few interesting mechanics in there that I'm going to borrow in for my own house game, though I'm probably going to mix-and-match different concepts, or even do both. (I like doing fractional resistance, as well as doing numeric reduction. Mix and match the two to create even more fun.
) And a number of these concepts had already appeared in one form or another in other places or houserules, or even as more limited versions. (advantage sounds like an extrapolation of the 4e elven accuracy.) Leaves plausible deniability when those bits show up in homebrew. >:}
I have to laugh, actually, because their hit-dice as mundane healing reserve is exactly what I'm doing in my non-D&D psuedo-d20 game Cityscape. (Fairly sure I even mention it in one of my posts on here, months ago.) Independent thoughts leading to the same result.
Monsters using different rules never really bugged me. (It's like using the Elite array vs the common 10s or something rolled, it's just how much player/npc differentiation are you willing to take?) I think they're going to ground the monsters in their stats a lot more firmly than they were in 4e.
I know in my own rules, I'm planning on building NPC classes that fit the various role archetypes of 4e, and having monster types only modify those, so like undead or dragons would upgrade hit dice by a size but otherwise leave saves/attacks/etc intact from the class archetypes.