In general I would say challenge. Not the challenge of the game but the challenge against other players/people.
Especially endless games like WoW are addictive because there is always something to do to become better, stronger, more respected and so on.

Many people also use games to escape reality.
Some do just for the sake of releasing stress or finding someone to communicate with. Others even see them as replacement for a real social life.

In Toribash it's the community. You can find internet friends here easily.
Hardly anyone on IRC still plays toribash actively so I wouldn't say they are addicted to the game itself.