Broad questions for the win? I actually shied away from writing an answer to this one when I read it. So many answers and I do not have the time to write the guidebook on this subject! It comes down to many things but behind that is the thing called effort and another little thing called belief. These are the things that will always make you achieve success if you are brave enough to commit to them.

Effort
If you want to make a success of something, you have to try. I know that it sounds stupid but the amount of campaigns that get thought of as opposed to the ones that get played to fruition is a horrific ratio. I listen to it every day in the store as people talk about what if’s and see the campaigns start at the table but peter out because the effort is not there!
I run successful campaigns because I realise what goes into them. I do not work on adventures with pre-scripted plots, I work with campaigns which have open endings and opportunities for stories. These are my campaigns. The other campaigns that I run successfully are the Adventure Paths and I succeed at those because I know how to be flexible and embellish where required, bend when needed and drive on if I have to.
I make an effort to think about what is involved in a campaign concept and commit to it only if I believe it can be done. Not only if I believe it can be done, but am I invested in finding out what that campaign will bring. Paizo’s Adventure Path’s are a perfect example of this decision. I do not even think of running one until I have the entire path and I enjoy the overall story. I then make it my own as opposed to just accepting what is in the book and I sell it to others. I do not run one of these without having an investment in it and also thinking that it is worth the player’s investing in it too.
Finally, the most interesting and successful campaigns are the ones that I design myself. These may run weeks through to years and may have one group of players or several groups telling the same story over an extended period. I have been telling one story in Earthdawn for over twenty years now and the final leg of the campaign is coming up with a new batch of characters in the near future whilst a Shadowrun campaign I have been running for the past few weeks may only last a few more. Both of them are successful concepts but it is the effort of myself and the players that make it a success.

Belief
Another thing with the effort is the belief that it is worthwhile. A lot of games fail just because this belief is not there. Some campaigns are just the idea of;
Hey that would be fun.
Without the actual belief that it is a viable thing to do. Some GM’s put out these ideas and do not get the buy in from the player. They have a concept of thinking that a campaign about X would be fun but not really find out if the players feel the same. Without player belief the game will die and without GM belief it will always fail too.
Success
This is such a broad question that I decided to attack it from a broad perspective. There are different concepts in effort and belief that I feel this blog has been answering for its many years of life that if you want to dig into the hows and whys you can by searching the back catalog. I run successful campaigns almost on a daily basis and I know that it is a lot of work to do so. I also know that it takes a lot of work from the GM and the players to do so. I know that a lot of that work or effort comes from the belief in a game and so I think that these two broad concepts are the heart of a successful campaign.