it's my ultimate comfort game. i've played hours and hours of this over the years. i forget about it for a while, and then inevitably return to it to play hours and hours more.
i think it's ultimate success is that every class feels fun and interesting to play - the systems allow for so many different kinds of run and playstyle to fit whatever mood you're in. sometimes you want to play a tinker and create a cool build with items; other times you want to play an orc assassin and stack conversion bonus until the dungeon breaks. it's so fun!