Generally I don't like "Save The World" campaigns for three reasons:
1. What can you do once the campaign is over besides retire? No encore.
2. Why isn't [Insert Elminster/Mordekainen Equivalent] personally involved?
3. The PCs generally don't care. Really. As RangerWickett mentioned, you have to love a world before you want to save it, so starting out a campaign with "Save the world" before your PCs know or love the world is not inspiring.
1. No, it's "saved". Its still there, needing saving again, from another threat. "Saving" is sort of subjective, rather than an objective measure. Someone's "saving" may mean another's destruction.
2. Elminister/Mordenkainen Equivalent are personally involved. They just suck at what they are doing to save the planet (PR's a b*tch when it's been too good...), or aren't attempting the same "save" the PCs are. In fact, Mordenkainen is probably the schemer behind the original plan, and Elminster his dodering geriatric tool with Alzheimer's.
The co-operation game is tricky. At high level's everyone is manipulating everyone else, jockeying for position, removing rivals, spreading misinformation, stealing credit.
"Saving" is indeed very subjective. I dunno about you, but my campaign doesn't run like an episode of the Powerpuff Girls.
Player's set individual goals. Player's recieve tasks from associated organisations/powerful individuals. These organisations provide suitable rewards for those that complete tasks for them, and/or are faithful to the cause. Players recieve more XP for achieving individual goals than killing X - looting Y.
"Saving the City" could mean rigging an election, or killing the mayor, or standing for election.
"Removing the Wererat threat" could mean killing all rats in the city, establishing a Secret Peace between the Lycanthropes and City Guard (with defined limits for "activities") or killing all the humans.
It is difficult to get players to think in terms of individual goals. Once they have them, then they start to "care". Caring, together with individual goals, gets players thinking "What do I need to do, to get the result
I want".