1) item sets
2) unique relics
3) re-fightable bosses (like the one in the old WZ)
4) multi-player void crusades & a better matchmaking system
5) general chat & a FAR better overall communication system
6) trading (ala Diablo 2)
7) a FAR better QA system...otherwise, you will continue to move two steps forward, three steps back.
8) a rigid, predictable development cycle (updates every [insert day/week/month]), accompanied by rigid, predictable announcements
8) FAR better character customization system (the current one is a joke, and the texture/color options are superficial, non-sensical, and almost lamentable)
9) more crafting/re-rolling information...trying to get a certain item enchant when there may be 0% chance is annoying.
I'm done for now...others can add some other ESSENTIAL features they would like to see.