I'll keep this entry simple. The ideal campaign - regardless of setting, regardless of genre, regardless of system - is one which runs itself.
A campaign that runs itself is defined as one which, after the initial setup, creates its own adventures.
What this requires is really just four things:
1. Loose ends. The dungeon, hexmap, city, etc., in which the campaign is set needs to be liberally peppered with questions which you, the DM, do not yet know the answer to. At some point, the PCs will ask one of these questions. Because you do not know the answer, the range of possibilities is open-ended, and has no conclusion. What does this symbol on a dungeon door mean? The PCs ask the local innkeeper. He doesn't know, but he tells them there is a witch who lives on an island up the coast who might. What will the witch want in order to give the answer? When they get there, the PCs discover her son has gone missing, and will give them their answer if they can help find him. The PCs discover the son was kidnapped by pirates... And so on.
2. NPCs/monsters with motivations. If most NPCs, dungeon denizens and so on are pre-prepped with motives or goals, adventure follows. If the gnolls are not just gnolls but have a rivalry with the local grimlocks, and are afflicted with a nasty disease, encountering them becomes not just a matter of "kill the gnolls, take their stuff", but a source of further adventure. And, of course, new motives and goals can arise. If the PCs kill some of the gnolls, or steal a treasure the gnolls had their eye on, the gnolls suddenly have powerful new motives.
3. Staying a page ahead of the players. Each session, afterwards, sit down and take notes about what happened. Come up with two or three hooks or events stemming from this. Make the world responsive. The PCs killed a nest of kobolds. Ok - next time they come by, the room is infested with scavengers, or dangerous mold, feeding off the corpses; or, Mama Dragon has come to find out why nobody attended her birthday party.
4. Don't be afraid to steal the players' idle musings and run with them. Players often speculate out loud about meanings, motives, explanations, histories. Don't make use of all of these. But riff on them - again, after the session, when you've had a chance to mull matters over.
The goal is to get to the stage that you only need to spend 10 minutes between sessions, essentially on caretaking tasks, like a well set-up garden that needs just a bi-weekly mow with the lawn and some dead-heading to keep it beautifully maintained.