It is understandable that you do not know the exact amount of hit points of an enemy when you are fighting, but you should still have a bit of an approximation of how good you are doing. So, I suggest that when you are fighting and an enemy reaches a certain percentage of health, you receive a message, such as "Your opponent is weakening", that way you don't spend all this time battling and never really knowing if you're even harming your foe or not
I believe better then that would be as Drogla said. To make it like once they have been weakened enough they start to have their posture change to a much more unstable posture. Also, maybe start to constantly bleed.
I think it would be good if the colour bar under the char (the f11 toggle button) showed health. So it would start out green, and fade to red, then you die.
This little tool could also be a major aid for constant powergamers. If you cntrl+click your opponent, you aim to harm him anyway. Whether he becomes a cloud or not shouldn't be something to worry about.