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.

I have to say,gameplay makes a game addictive to me.Gameplay makes a game not its graphics or achievments.
Lets say for example,Stronghold crusader.I like that game even though the graphics sucks.
Age of empires is awesome too and its graphics are not that good.

sandbox shit, create-your-own shit

Minecraft,
City of Heroes,
Toribash,
GMod (my account got hacked )
i like to be creative, etc.

Gameplay, any game where the character can look cool doing something. Or the content within the game.

If the game is fun and you could play it for over an hour without getting bored then that right there is an addictive game. Some other things I look for are reviews, graphics, and blood and gore.

Personally, I like a challenge. Achievements are also good as they give a sense of success. Of course, a game with achievements and that's challenging and has good graphics will over rule a game with all the same and bad graphics.

Coffein

different kind of people like different kind of games ...
People not able to do most of things in real life choose games fitting they're imagination of themselves ;)

an online game that makes you interact easily with other players, with good factors that are related to the real world (market, shop, art....)

I think you know what game i'm talking about ;)

Gameplay.
The way a game is played, the way we can interact with a game and the things the game lets us do determine if a game is good or not.
For example, toribash and minecraft.
These 2 games are very addictive because they're different from every other game out there.
They're not shooters, not football, not fighting, not cars.
Variety is also an important factor in all this.