In my experience, the thing that makes or breaks a game is compatibility. The players must have a compatible (though certainly not identical) style of play, the GM must have a compatible style and purpose. I’ve been rather unhappy as a (temporary) player in a game where the focus was on statistics or on the Underdark, and I couldn’t give a flying crap about either (not that those things aren’t worthwhile, just that they don’t interest me much). On the other hand, I am currently playing in a truly great game where my style of play matches that of the other players, and the GM’s purpose in running the game matches my own creative interests.
The reason I ask, heart in my throat, is because, as a long-time player and only occasional GM, I have taken it into my mad wee brain to perhaps run a long-term campaign. Thanks!