The most commercially successful games all have a default answer to this question built in to the system:
How will play in this same campaign 8 months from now be fundamentally different than it is tonight?
In D&D the default answer is: you'll be levelled up by then, and this means something. You'll be fighting crazy godlike monsters and exploring planes instead of dungeons or be building castles. Here is the concrete evidence: the spells go up to level 9 and the monsters include like Tiamat and Demogorgon. Look, there it is, waiting for you...
In Rifts the answer is: you'll either be fucking with this Coalition that rules North America or you'll get to see some whole new continent full of more crazy robodemonborgs. Proof: look at these crazy splatbooks we keep putting out Japan, South America, Africa etc. Unlike D&D, your PC will not have changed that much, but you will be seeing new stuff, so keep on.
In Vampire: TM the default answer is: you'll be higher up in the convoluted Vampire hierarchy and/or you will be further along in the metaplot that you've noticed we keep publishing. See, it keeps coming out. Even though you will still be pretty much some bloodsucking badass like you already were, the story will have progressed to some new phase.
In Call of Cthulhu the default answer is: you'll be dead or insane. Or more insane. Or insane in some new way. Or you'll be surrounded by dead or insane friends. Anyway point is the situation will have become entertainingly more desperate.
In Warhammer the default answer is: you'll have a whole new (better, more badass) career and you may also be insane. Or mutated.
-Shadowrun: possible exception?
Point is: every RPG session produces changes in that session. "Bangs" "assignments""plot developments""twists""occurrences" whatever.
The really successful games promise or suggest (though do not mandate) a specific kind of change will occur over the long haul.
This is like a barely perceptible carrot keeping the players interested session after session, even when they don't know it. The game is going somewhere.
Notes for GMs:
-Not all level-up systems automatically produce this idea of change. In D&D, level 20 is not just level-1-but-more-powerful, it's practically a different game. You go through distinct phases (usually) in D&D: useless schmuck, badass hero, crazy godling loaded down with items. In other systems, levelling up just means you are kind of better--but the enemies are still the same relative to you.
-The game ending is not an expectation of change. In The Oddyssey the sailors expect to be done and not sailing any more when they finish. That's not what I'm talking about. If they thought in 4 months their boat would be flying and four months after that they'd be maybe going through time then that would be what I'm talking about.
-If you remove a game's built in "change engine" or it doesn't have one but you still want long-term campaign play then either: the players have to trust that the supply of new and crazy exciting ideas you are coming up with is basically infinite (which you may be able to guarantee but the system can't) or you have to create (or let the players create) a developing plot that intrigues the players and makes them want to see what's next.
-Example (thanks John): a D&D ranger guy who is doing a land survey, mapping uncharted territory. If the player thinks genuinely that the next territory will be new and interesting, that is an expectation of change over time. There you go.
Or: A Conan type trying to conquer stuff. If the player expects to conquer stuff and then the game ends, that isn't the expectation that'll keep tons of people interested. If the player expects to conquer stuff and then the game changes to where it's about armies and castles and new lands--that's a change over time.
Smart GMs seems to set up these expectations automatically--like Jeff Gameblog has a dungeon full of 1st-4th level PCs and he is already talking about the dragon in there and the wizard that built it and the possible portal to Hell in the lower depths.
This also may be why lite indie games have a reputation for being one-shotty. Unless the players (or GM, when there is one) makes up a specific change-over-time expectation, the games usually don't provide them on their own. (I have no quarrel with Noisms comment below about how this is often a feature and not a bug.)
Now in the comments today we will inevitably meet someone who goes "Oh I played such-and-such for 80 years and nothing ever changed and my whole group loved it". That's great. We believe you. Pat pat.