Group/Team Playability
Usually i solo in games, but its always good to have certain places where you can't solo, and also if the game has certain benefits for being in a group (e.g. increased exp, drop, auto pick up etc)
Player Vs Environment
Every game needs a good PvE, unless of course its pure PvP based lol, but when im fighting mob i do prefer it if the mobs of your level dont drain 10-20 HP/MP pots per kill lol
Graphics
Only the graphics of skills interest me tbh, i always need a bit of eye candy in my games (With the exception of mir)
Player Vs Player
Always love a good PvP, but the game needs to have even classes so that everyone stands a chance against everyone else (Of course with similar levels and items lol), the game also needs to have a PK system and dualing system. The PK system, obviously you all know how that works, but the dualing system must disallow pots, others it could go on forever :P
Amount of players
Depending on the game would determine if it needs a good player base, e.g. a game like Fury needs a good player base, since its based upon PvP, and the games are activated when a certain amount of players apply to the arena lol.
But hey, the more players the better i guess since you can find items to buy a lot easier, sell items a lot quicker and party more often