What Makes a Successful Campaign?
First and foremost, I can't be GMing it. Seriously, I haven't had a game go past three or four sessions in more years than I care to admit. Now, to be fair, I'm not wholly to blame. My last few attempts have fallen afoul of work schedules, holidays, and an unplanned hospitalization. But after a while, it starts to weigh on a person.
So, with that in mind, I do have an understanding of what it takes to make a campaign work, even if I haven't succeeded in a while:
GM Dedication: This should be obvious. If the GM is unfocused, or uncommitted, the game is going to fail. Maybe not immediately, but sooner than later.
Player Buy-In: The player's have to be on-board as well. For the campaign premise, for the rules system, for making time to play, and for spending hours in the company of the other players.
A Regular Schedule: Every game I know that works long-term has a schedule the players and GM abide by. It's a part of their routine, not something they squeeze into their busy schedules. This is an aspect of dedication and buy-in, but it's worth calling out separately.
Rituals: My Tuesday group meets for dinner at 5:45 (having hashed out the location earlier), eats until 6:45, then drives over to our venue (Rice University) and play until 10 PM. Meeting and eating helps us get the socializing and cutting up out of the way, but it also provides a nice secondary bit of continuity with the group.
Communication: Do I really need to say more about this? Nah. Communicate, nerds!