When any mob has a buff on it, you really don't know that it does, or what it has, unless you are paying very close attention to it, or if you hit "examine", that little magnifying glass icon.
But who has that kind of time in the midst of combat?
My suggestion is that when a mob casts a buff on himself or another mob, there should be, say, some kind of mini-icon or glow, depending on the type of buff.
Is the ogre raged? Give it a reddish aura or glowing eyes.
Does the drow priest have deathward? Give it a broken skull icon.
Does the red-named fire elemental have 'Protection from Energy: Cold' on it? Give it a blueish shield icon.
These icons don't even have to be three-dimensional, as long as the player can see them. Also add in a check box in the options menu to switch this on and off. It would also help if some changes were made to make debuffing a more practical strategy, like changing how dispel works and giving cooldown times to caster mobs.