What makes a successful campaign?
First, I'm getting a little behind in this. Time to catch up.
For me there's a moment in some campaigns when the group stops viewing the campaign as a series of adventures or challenges and actually lives in the world for a moment. If it happened in the D&D campaign I recently ran it was when the group had the PC's sitting around the Inn of the Seven Winds talking about how they might turn around the failed economy of the city of Grimfest if/when they liberated it from the gangs who were at the time running the city. In the Marvel Heroic Campaign it might have happened when the group began to converse with their live-in NPC Mrs. Robot about her post-transformation social life and ate cookies than she had made for them.
If you can transcend the challenge/achievement mindset that makes RPG's little different from board games and have them imagine themselves living in the world, even just for a moment, that's a successful campaign in my mind.