What makes a good campaign?

[Rainbow Servant]

So I've been wondering lately - what makes a good campaign.
If you were to break a campaign down into various pieces - say to allow multiple people to work on it at once, or to allow for small changes to give replay value, whatever - which ones would you say are the most important?

For example:
Campaign One: The Campaigning
Setting: Generic
Genre: High Fantasy
Adventure Hook: Mysterious wizard comes asking for help
Tone: Mildly Serious
Scope: Teh Epicz - Lvl1 through 20+
Hack n Slash: Present
RP: Present
Puzzles: Lite
Fleshed out NPCs: A few important ones, but Innkeeper 334 looks a lot like Blacksmith 519.

Campaign Two: Eclectic Bungalow (wait, that's not right...)
Setting: World made out of cheese
Genre: Private Investigators
Adventure Hook: Someone stole the mayor's rubber chicken!
Tone: Silly
Scope: Misanthropic adventures in your hometown (low level)
H&S: Not so much
RP: You betcha
Puzzles: Sure, but they all get set on fire.
NPCs:

And so on.
In short, think of your favourite campaign you've played in - why was it your favourite?

Edit: To be clear: Which factors are most important (and examples as to why would be nice plzkthxbai)

[Cyrion]

I think the most important aspect of any campaign world is that it be robust enough that you can accommodate adventures that the party WANTS to have (in character and as players)- give them a chance to have their own ambitions, goals, etc. and to have fun along the way. This idea is geared toward campaigns that span multiple adventures as well as multiple sessions.

[valadil]

The best campaign is one that I can leave my mark upon. I don't care how detailed or well crafted it is, if I can take two completely different characters through it and they have the same experience, the campaign is not one I'm interested in playing. I want a world that reacts in a unique way to the characters that are put in it.

[Tyndmyr]

The best campaign is one that I can leave my mark upon. I don't care how detailed or well crafted it is, if I can take two completely different characters through it and they have the same experience, the campaign is not one I'm interested in playing. I want a world that reacts in a unique way to the characters that are put in it.

This. So much this. I've also heard it said that a good roleplaying experience is a series of interesting choices. Choosing how you will affect the world is huge.

Players always tell stories about that one time, their character was worshipped as a god, or unleashed x, etc. Interesting effects on the world are one of the things that easily translate as a story without requiring system knowledge, and is often actually interesting and memoriable. Much more so than the guy who tells you how awesome is character was and how much xp he had.

[Zaydos]

The best campaign is one that I can leave my mark upon. I don't care how detailed or well crafted it is, if I can take two completely different characters through it and they have the same experience, the campaign is not one I'm interested in playing. I want a world that reacts in a unique way to the characters that are put in it.

Another vote for this. The campaign of mine that my players really enjoyed was the one where they became a duke, changed the course of a war, saved the world from illithid invasion (the actual main quest), started a wizarding academy, and raised a rust dragon to be good.

[Kurald Galain]

I'm essentially agreeing with the above. Being given meaningful choices that make a difference in the campaign world is what makes it memorable.

[Kallisti]

I'd say that a good DM is critical.

As the above have pointed out, the story is as much about the characters as it is about the plot. I'd say that's more on the head of the DM than of the campaign world--if the world is too restrictive by nature to allow for interesting characters to interact in interesting ways with the world, he can always alter it.

Depending on the kind of campaign, good NPCs are important as well. If you want players to be immersed, you need to provide realistic people. An unrealistic world is not really too much of a barrier to immersion--an RPG with a completely realistic world would be pretty boring--but people are people regardless of if they're the Spring Maiden of a Changeling Court or what-have-you. If you're less worried about immersion interesting NPCs are all you need, but otherwise you need interesting and well-fleshed out NPCs.

I'd say those are, for me, the two most important aspects of a campaign/campaign world, and both of them are, in turn, aspects of a good DM.

That said, it's perfectly possible to have fun in a campaign that doesn't fit those standards, although I'd say it's much harder if your characters can't leave a lasting impact on the world, or at least on their little corner of it.

Tl;dr: What the above said, +a good DM and good NPCs.

[Rogue 7]

A campaign where the players have fun.

[Emmerask]

I like deep involving story campaigns,

Campaign One: Borbarads return
Setting: Aventurien (dark eye) mashed together with d&d 3.5 (yes it works quite good)
Genre: mid/high Fantasy
Adventure Hook: prophecy
Tone: Serious++
Scope: Lvl1 through 20+
Hack n Slash: rarely
RP: lots
Puzzles: lots
Fleshed out NPCs: I think well above 40 books full of npcs, cities etc...

[Choco]

A campaign where the players have fun.

A campaign where the players AND the DM have fun, and refer back to it fondly for years to come.

[Lapak]

The best campaign is one that I can leave my mark upon. I don't care how detailed or well crafted it is, if I can take two completely different characters through it and they have the same experience, the campaign is not one I'm interested in playing. I want a world that reacts in a unique way to the characters that are put in it.

This, with the additional condition that it has to be the style of campaign that both players and referee are happy playing. If I don't want to play a super-political game of Vampire:tM, it doesn't matter how much of a mark I could make doing so.

So, two qualities:
1) Player agency
2) Overlap of player and referee desired styles
are critical.