My experience is similar to [MENTION=23240]steenan[/MENTION], [MENTION=6683099]dd.stevenson[/MENTION], [MENTION=6688937]Ratskinner[/MENTION] and others - a long running campaign needs a conflict that the players are engaged in (via their PCs), and that means that the conflict has to be something that speaks to the players. I find this is easiest if it's a conflict that they have helped to build, either through PC backstory or in the course of play: I tend to find that my campaigns take a little while to warm up, as I start with small conflicts that are immediately salient to the PCs, and then - as the backstory becomes richer and the players' interests better defined - these can be linked into the broader themes and concerns that become the focus of the mature campaign.
For me it is not as simple as good-vs-evil - often my players (and their PCs) aren't all that sympathetic to the official good goods or rulers, and have their own views on how the world should be arranged - but the campaign has more energy when the concerns of the PCs go beyond their own narrow self-interest.
Ratskinner is also right about the MEGO problem - one of my long-runnning campaigns came to an end because of this, as it got to the point where even I (as GM) couldn't remember or make sense of the main antagonists motivations, and hence the PCs' reasons for opposing him.
For me it is not as simple as good-vs-evil - often my players (and their PCs) aren't all that sympathetic to the official good goods or rulers, and have their own views on how the world should be arranged - but the campaign has more energy when the concerns of the PCs go beyond their own narrow self-interest.
Ratskinner is also right about the MEGO problem - one of my long-runnning campaigns came to an end because of this, as it got to the point where even I (as GM) couldn't remember or make sense of the main antagonists motivations, and hence the PCs' reasons for opposing him.