The single most important factor is one I'd take from gaming, a medium where addiction is a frequently sought-after element: Playability. Do things flow in a pl

The single most important factor is one I'd take from gaming, a medium where addiction is a frequently sought-after element: Playability. Do things flow in a pleasing way? Do your actions feel like they have weight? Is the environment responsive? Do you feel like you can explore? Strive for a "playable" UX and addictiveness will come naturally.