What makes a successful campaign will vary hugely based on the people you ask, everyone looks for something different in a game. I have friends that just play for the social aspect, to see their friends and have a little fun. I have others who see roleplay as an almost theatrical experience and others still that see it as an extension of wargaming and I guarantee every single one would say something different if asked what makes a campaign successful.
For me, I fall somewhere in the middle of that triangle of gamers above. Gaming is a social thing for me, I enjoy seeing my friends but it’s also very much about the game for me and I take the game very seriously when it’s presented as such. A successful campaign needs to be run seriously, by a DM who cares enough to do so and puts the work in. If a game isn’t presented that way then i’ll play and have fun, but never as much as if I can properly sink into a character and a game.
I think least of all I see roleplaying as a wargame but it retains some elements of wargaming and a successful campaign bears that in mind, especially one that involves an element of combat. Players want to be challenged, to not feel as if they won too easily and to have the satisfaction of besting their foes.
Ultimately though the story is the bit that matters, how the DM presents the world, the plot, the characters, everything. A successful campaign needs to feel real, I need to hate the villain, care for the side characters and desire to see the story through, beyond the general desire to play the game and pick up the plot hooks.
A successful campaign is the very definition of roleplaying, at least in as far as it’s described in many modern RPGs, it’s a group of people working together to tell a story and the more successful that endeavour is the more successful the campaign is to me.