My concerns might be that focussing attacks on individuals would become too powerful - it might be easily possible for monsters played well to take out the party leader in one round, before players have a chance to do anything.
I've been doing a lot of focus-fire lately, particularly against the back rank (with one monster distracted by the defender). One round of attacks is generally enough to bring a (5th level) PC just barely away from death. It's so consistent I think the system was balanced for this type of combat. It ratchets up the tension nicely, too.
So if you're cutting HP in half you'll have to be very cautious about focusing fire on the PCs, I think.