One where new things can happen every new day not the same old grind. Interactions with players that makes things more interesting, not just simple variations of different events. One where I can go "LOL did that just happen", or maybe discover something brand new that was carefully crafted and well hidden from the regular denizens trotting upon the game's land. If I play a game seriously I'm pretty much always in the very top 0.01% or so of the players so it really takes a deep game to keep me interested for an extended period of time since I just master one (as in concepts, understanding game design, playing to top 10, and other multiple aspects) and move on when it gets boring.