Conquering everyone is sometimes fun, and I'd enjoy diplomacy if they didn't shanghai us into becoming the United States during the end-game (yes, I am a US citizen, but I STILL think that's lame, cultural imperialism anyone?), but I have to be honest, if you haven't conquered everyone then the only way to "win" is to have the highest score in the time victory.
So what if you flee my super advanced highest all around score culture in a space ship? In a little bit I'll follow you and still be better than you. So what if you have three great cities if my whole civ is beating you to death in general? Eventually I'll take your three legendary cities because I'm just that good. If I'm still around, and I'm still all around better have you really won anything?
So when I win those ways it's cool, but I still don't feel like I've really won the game, unless I also have the highest score, in which case I might as well play on and see if I can KEEP it until 2050 anyhow. Of course I understand a lot of other people feel otherwise. My best friend thinks winning by score is weak, for about the opposite reasons.