Neonchameleon
Legend
I think that the designers seem to be doing well with that in 5E so far: the really important things are to make a simple system that is easy to use, give everyone something cool to do, and then step back and let everyone have fun.
For the record I'd agree with this - I'm actually running a campaign in 5e with the latest playtest and for the most part, the rules seem pretty good at getting the hell out of the way and letting the game progrees; the first edition of D&D I'm aware of that makes this close to happening RAW (4e took the opposite approach and made the rules fun).
AD&D experience, though, hits OMG at name levels, and it can take a year of play to go up one level after that, you can run games for a long, long time, and, while some classes top out, and others get basically nothing for leveling after a while (ooh, 3 hps), some have spell/day charts that go over 20, and can clearly be extended ad infinitum, if you so desired. So a campaign - particularly a campaign made up of humans that aren't Druids, Assassins, Monks, or Bards with maybe a few non-/demi-human thieves - can quite literally go on forever.
AD&D enters the endgame after 10th level. You get castles and followers and are meant to change the way the game is played. And there's a reason the highest level PC at Gygax's table (Sir Robilar) I'm told was 14th level. Also if you follow the 1e rules (2e relaxed this) all the non-humans went home long ago.
Plus [MENTION=29398]Lanefan[/MENTION] houserules for a long campaign. I assume most of the other really long games do too.