Here's my list:
1. Gameplay: how it plays, what you can and cannot do.
ex. GTA:SA compared to Halo 2, both have different perspectives, different weapons, different vehicles, different weapons, etc.
2. Multiplayer options: Single player, Multiplayer, Internet, whether it's Pay to Play
ex. WoW compared to WC3, both have multiplayer, but one needs to pay
3. Fun factor: Is it fun?
ex. Any game compared to chess or minesweaper :P
4. Longevity: how long I'm able to play without getting bored, depends on how fun it is.
ex. A game with one plot compared to a game with multiple storylines.
5. Interactable: Is the game static, or can gamers contribute to maps, weapons, mods, etc.
ex. Halo 2 maps compared to CS maps
Things that are not as Important:
Graphics: How it looks
Sound: Soundtracks, sound effects, etc.
Characters: Main, side, etc.
Plot: Main storyline.