PS's RP
WoW's Game mechanics and environmental shtuff
T4C's storyline
Runescape's economy (Believe it or not, RS has a well-balanced economy)
SWG's vehicles, and expansiveness
Guild Wars's Graphics and PvP
THAT is the ultimant MMO 
No I have an even better one(Though more complicated), a MMORPSG(Massively Multiplayer Roleplaying and Strategy Game) that includes the previous ones and also:
- A very mature and roleplay oriented community.
- Yearly cyclic age shifts based on RW years and each year every character is wiped for the sake of realism(1st year is Medieval, 2nd is Renassaince, 3rd is Imperial, 4th is Industrial, 5th is Modern, 6th a cataclysm happens and puts the world back in Medieval -- cause futuristic stuff is excessively explored by other games --, and so on). Something like a MMORPG version of Civilization series.
- Each year is also a contest for those interested in Empire Building, "Imperial Wars" will be epic in Medieval Ages, and very large scaled during modern ages, like WW1 and WW2. Also there will be less violent and more discrete ways of conquering lands. Manage an economy while you roleplay your character dealing with diplomacy, etc.
- The winner of the previous year will hold a legacy for the new development age, but still it will be small enough to allow others to overcome it, and according to the type of empire victorious, other things like civil unrest will exist to make the competition fair.
- Governments stability are fully based on the competence of the players who manage them, revolutions(specially on the Imperial and Industrial ages) will cause constant shifts of culture and social values that will erupt into massive civil wars as the fallen governments attempt to counter it.
If such thing existed it would be the game of the century!
