In some classes we tried to make a brief list of what makes a game great. I have my own ideas (which I briefly discussed in my contribution to Hobby Games: the 100 Best), and I intend to write a separate article about that, so I'll just list what we came up with in class in no particular order.
Innovation--it's good to be first
Improving realism (e.g. destructible environments Red Faction) (bullet time) (scars) (camera movement) etc.
Really good gameplay (playability)
PvP, team play--something other than person vs. computer
Variety of modes of play (online, pvp, MMO)
Verisimilitude of the environment--when you’re there, it feels like you’re really there
Your character can grow and change (skills, attributes)
Good interface and tight game controls