I feel like I'd be better placed to answer this if I'd ever run a successful campaign because then I could comment from both sides, and talk more technically.
What makes a successful campaign?
- A strong storyline. The GM needs to either have this ready or, better, to plant the seeds and nurture what the players grow.
- Developed characters who are engaged with the world they're in and who have ties with the characters - best if these ties are created or expanded during the campaign.
- Personal as well as party as well as global plot.
- A simple system that doesn't distract from the campaign with lots of intricate rules questions.
- An understanding that not everyone is going to be at their gaming best every week.
- A routine in terms of where and when to play.
I think those first three are the most important and are also tied together: the GM knows the world and what's happening behind the scenes, but if the players don't bother engaging with it (even if that engagement is to find ways to ignore the monsters to comic effect - if that's what you're going for) then it's wasted: the characters need to feel part of the world, and whilst the GM is primarily responsible for creating and overseeing that world, it's down to players and GM to keep that engagement. The players need to work with each other to create the relationships between the characters, and with the GM to build relationships with NPC's, and the GM needs to reward that with those relationships and with plot.