(No, this isn't going to be a what-if featuring Jane Austen playing D&D. -ed) (Hey, that's not a bad idea! -B) (No! -ed) (C'mon! -B) (Dammit, I said no! -ed)
Warning: This post will contain links to game wikis. If you are worried about spoilers, I suggest not following those links through.
So yeah, this issue's been bugging me for a while, and if I can't get a suitable discussion out of it, I can at least get some content in here. First of all, what makes a game re-playable? Using some of our standard examples, in Kingdom of Loathing, once you beat the final boss, you are given the option to ascend and start at level 1, keeping one or more skills from your previous run, and doing the new run in a variety of different ways. In Fallout: New Vegas, and the other Fallout games to varying extents, you can play through using different skills and/or make different choices, leading to other endings. Another option used in New Vegas is that of achievements, giving you some sort of boost to your fragile ego by giving you awards for things that you either might normally not do, or for doing a lot of the things you would usually do. If you have a couple spare hours, you might ask Zerg_Rush how he got the title "The Insane" for one of his WoW characters. I'm sure you've run across these tactics to squeeze more playtime from other games, I'm just using the examples I've run most recently. Some, of course, are more effective than others.
The more philosophical question is what actually causes one to replay a game? Havoc Jack just spent at least one post talking about how he's restarted Diablo II, but I can't normally seem to muster up that much desire to replay a lot of games, especially after I just finished it, whereas I've seen him finish a Starcraft I campaign and immediately start replaying the same campaign. Every once in a while, I will go back into the vaults and work up some game I haven't played in a while, but for the most part I don't replay games that much. Starcraft II was pretty awesome, but after playing it once I haven't really gone back to it at all, except for running a few missions trying to get achievements. I haven't actually finished Fallout: New Vegas yet, but I don't really plan on playing through again once all the DLC comes out and I actually do finish the game. I ran though Fallout 3 twice, although that was primarily because I got DLC for that and decided it would be simpler to play through again. On the other hand I play KoL all the time, and am constantly ascending. Perhaps it's just that easier accessibility to a wiki and easier character optimization makes me want to play it more, although according to Steam I have played New Vegas for 62 hours, so I guess that's a decent amount of time spent on it. And of course, I did end up reading a lot of the wiki, and sometimes I find that reduces my desire to actually play the game.
I'm not sure why this single-play tendency only applies to video games, though. I've played any number of board games any number of times, I've watched many movies as much as I could stand, and nearly all the books on my bookshelf have been read dozens of times.
Insofar as no one else can answer the question for me, perhaps if anyone actually reads this blog instead of it being a vanity project for us they can attempt to answer the question for themselves.