I thought I'd repost my concerns/issues with this type of system, from the other post, here...
I could see so many problems with doing it this way...especially if it works for both players and monsters.
My main point of contention is the metagame aspect of it all. How does a player know when a monster falls below a certain hit point threshold? I certainly don't give out this information in the game's I run, so I see two possibilities from a player perspective.
1. Tons of wasted spells as they guess at what hit points a particular monster or creature is at.
2. Metagame is now the game, Everyone(DM and player) knows how many hit points their opponents have and act accordingly in the game.
Neither of these options appeal to me at all.
From a DM's perspective it also has certain problems. First in order to use abilities that are dependant upon what hit point threshold my players are at I will have to cross reference their hit point totals and the total at which the ability takes place throughout the combat. I don't see this as streamlined in anyway, it seems overly complicated since each PC is going to have different hp's at different times throughout the entire encounter. Second, how would my monster/NPC know when to use this ability? Yet, I as DM will always know exactly when to use it. Do I pretend the monster is at the same disadvantage as the players, or are we once again at the point where metagame is the game.
IMHO, this would be jarring in the extreme during play and would have the added complication of both PC's and DM's concerned with tracking their opponents hp's. I can see it now, everyone with a small tablet steadily recording how many hp's each has so they'll know when to use their ability.