(on group quests for SP)
you'd probably need an NPC
Beware! This is where madness lies. At least for developers foolish enough to try it.
Maybe just put HP bars on bosses...Although, I would not prefer it myself.
That solution makes it even more arcady. "Look, this is a boss!! He is special because he has a HP bar".
I think the HP bar is because of one single feature: Regeneration. Any normal monster can be killed by just shooting/hitting long enough. If you encounter it again you know if its worth doing it.
But regeneration pisses on the parade (lots of reference to bladder function lately. Is this a trend?). A beginner has no realistic chance to find out about regeneration except by reading about it in a forum/wiki. Books and Info pages inside the game are unreliable as well, many people don't read them (their fault but a game developer has to account for that) or don't find them.
So what would be a good in-game way to inform a player that regeneration exists? I can think of a few:
1) You find a corsett of health regeneration. No, that is not magic at all, it was developed by a mad scientist and it simply injects you with very low doses of glowing zombie serum whenever you need it (indicated by low blood pressure and pain nerves signals). Side effect is you start to glow. Now this should give the player a hint "glowing=regeneration", also the description of the device should practically give it away.
2) Instead of a corsett make it a serum (and make one ingredient part of irradiated zombie loot)
3) Naturally books are a nice way to inform the player, it is just difficult with random loot to make sure the player sees it. Maybe a quest on the main quest line should always provide the book that tells of dangerous regenerating zombies.
4) if the health of the zombie is shown as part of the texture and/or posture, that would be ideal but needs lots of work from the graphics developers.
5) Remember the Fallout 3 quest "Writing a survivor handbook". This is the quest which made me like Fallout 3, whatever shortcomings it had otherwise. Ok, simply copying the idea won't work well, but something similar could make a nice continuation of the starting quests. Example: Some scientist hires you to find out stuff about the zombies. Can they reproduce? Is there still blood in zombies? Do they pick up stuff or is their loot always stuff they owned while alive? ;-). And ... how do irradiated zombies regenerate? Finding out about that could be a cool quest to go into higachi tower and find out more about the research done in there.