From a GM perspective, I need a concrete sense of how progression works in regards to the characters and the world. I want to know pretty solidly what 1st level, 4th level and 9th level characters are like before I even start. If there is too much randomness in progression, or progression is too shallow or too steep, it makes me think it isn't a good long term game. Same with extremes of rates of progression.
Tied to that is having robust and interesting widgets -- monsters, traps, tricks, etc-- at all levels of play. If there is too distinct a "sweet spot" from a GMs perspective, I don't consider it good for long term play.
Finally, the game itself should support lots of different kinds of adventures or other things the PCs can and will do. If that is too narrow, like a game built for combat and little else, or just dungeoncrawling, I don't consider it good for long term play.