Diablo 2 did try to remove that when you faced Diablo himself. His health regenerated quickly when he was idle so by the time you got back to him, he was at full health again.
Sorry Handefood, just plain wrong. End-act bosses don't regenerate health.
Not all the bosses, just Diablo. I might be wrong though, it's been a long time. I think I read that in the Official Strategy Guide which is based on v1.5. They might have changed it. I'm pretty sure he does regenerate as I remember him being a little bit healthier everytime I came back.
