Gameplay and multiplayer features are on my top list, ever.
Followed by re-playability and the existence of modding tools and patches to enhance the experience with the game.
The plot and the elements that makes you re-play the game countless times differ from genre to genre, but all of them share at least one thing in common: good gameplay.
Baldurs Gate, Diablo, Age of Empires, Starcraft, Warcraft, Half life, Counter-Strike, Quake, Unreal Tournament, Chronno Trigger, Final Fantasy, and the list goes on...
