There will always be a strong element of personal taste involved in these things. To a degree an active large selection of players is best, but sometimes people want to experiment with a different and potentially different quality of play.
For instance, I am a junkie for character advancement. However, I think rewarding fun with advancement is detrimental.
Power creep changes the scale of character activities (you go from conning goblins to conning dragons or invading empires as an example). If the players aren't able to create stories at the higher level, or can't relate and reconcile their actions and consequences with others, then your scale is too high, it is unsustainable.
Or you get The Reach, where hundreds of Epic Heroes languish with epic birthday parties, and scrabble for a scrap of relevance or contribution to things labeled "beyond the reach of a few/many heroes". Meaning your growth is meaningless, and it cramps you from having player created content.