What would be fun for me in a MMORPG:

1) A massive procedurally generated world to explore beside the usual areas.
2) Unpredictable mob AI (in battle).
3) No EXP leveling, but individual skill leveling instead. (you level what you use)
4) Huge tactical advantages for team play (like FFXI job system).
5) PvP.
6) True player driven economy.
7) Crafting based on difficulty to obtain materials, not timesink mechanics.
8) Interactive environment.
9) Random or triggered server events and/or world alterations. (players farm sheep too much, a giant ram HNM pops out in the zone, for example)
10) Difficult timed instanced battles to test teamwork and individual skills.