I was playing my witchdoctor (lvl 51 at the time) during Act 4 Nightmare mode. While some mobs gave me exp (such as the morguls and the corruptors) other demons were not. In particular i refer to the shadow spawns, the creatures that spawn when you kill a festering corruption, and the mobs that drop out of the sky and crash like meteors before attacking you (can't remember the name)
In the case of my wizard (lvl 9, Royal crypts Act 1) I was gaining no exp at all from standard skeletons, yet more advanced mobs like the skeleton arches or the shieldbreakers still gave experience.
Ultimately I'm just wondering if in both cases my toons were too high (in concern to level) to receive exp or if it is indeed a bug, thank you.