As I write this, I am knee-deep in my sixth playthrough of Fallout: New Vegas. The mere ability of a game to hold my interest for more than two full campaigns, let alone five, is an amazing feat. When you add to this that a single playthrough is, for me, months in the execution, then the lack of fatigue and eagerness to jump into the next expedition is truly a sign of this game's brilliance.
So what keeps this game from going stale? It's a well-executed combination of factors. The simple freedom that the game gives to go in literally any direction at any time ensures that no two games will be the same. I'm not just talking about the ability to choose between NCR, Legion, House, and Yes Man—in fact, now that I have gone through all the endings and gotten all the badges, I find that I like the independent, Yes Man route and have adopted that as my preferred path. But even so, the character of each game will be radically different every time; in fact, it seems impossible to play your next game through anything like the last one, even if you try.
Building upon the solid foundation of the game is a very active modding community. Credit for this goes to both the modders themselves and to the game designers for making the game so readily customizable. Many of these mods are of nearly commercial quality themselves, and once you've tried them, you'll never want to play without them again. Talk to anyone who has significant play time under his/her belt, and you will most likely get a laundry list of favorite mods that make the game even better than it already was (I won't bore you with mine—everyone's experience will be different anyway).
Of course, all this would be pointless if the game wasn't just plain fun to begin with. It's easily adaptable to a variety of skill levels and techniques. It forces very little on you; just the opposite, it begs you to play it the way you want to. And while I understand the appeal of multiplayer gaming, I think that the Fallout series' "single-minded" dedication to the single player aspect has been the right decision, by allowing the designers to focus on the player's experience without adding the overhead and bringing in the compromises that would be necessary for multiplayer (how would V.A.T.S. work in a multiplayer world, anyway?).
So let me conclude this little diversion from my latest playthrough with a tip of the hat to everyone involved in this wonderful game—be it the original designers, the mod community, or the people who contribute to the wiki, forums, etc. Everyone has been a part of something grand and utterly enjoyable. And with Fallout 4 somewhere on the horizon out there, the future looks bright as well.