It's a bit more bookkeeping ("did you remember to apply your armor HP bonus this round?" and "What's 5 times 7 divided by 2?"), but that's actually quite cool.
Of course, it makes characters ridiculously easy to hit (just like all armor-to-DR or armor-to-HP systems), and that whole "don't give a damn about AC" problem kicks in at even lower levels than before, and certain players low-level players may not enjoy giving up a 30% chance to avoid all damage in exchange for 3-12 HP.
I'm sorry, but I'm a little confused; how often does this temporary HP pool refresh?
I think it's per-round.
It seems like you saying that every round (at level 20) your parties tank would need to take 130 points of damage before you even scratch his regular HP pool. Also, if you don't get the temp HP down to zero (like you only hit him for 128 or something) to break the armor, does he start the next round at 2, 130 again, or some other number?
I would assume I'm just misunderstanding this, because if not then I think it's a little OP.
At level 20, a disintegrate
(3 spell levels behind what you should be casting if you want to be scary) deals 240 damage on average, blowing beyond temp HP and dealing over 100 damage besides. No, it's not all that OP considering what's out there.