My main points are thus follows;

Community generated stuff. (mods, weapons, hats)
Updates for free from developer (none of these silly pay-for-map packs)
A mature audience (nothing is worse than hearing a pre-pubescent child over a mic)

The only games I played online were TF2 and Killing floor (mainly because my comp is not the strongest of the bunch). TF2 is a good example of a good game.