I'm very picky when it comes to PC gaming. Seeing as how my budget for games, period, is fairly small, when I buy a PC game I look for two things. The first is a game that I'm going to enjoy. Yes, there's flight sims and extremely intricate RTS'es that have you micromanaging your units down to the style and color of underwear that have gotten rave reviews, but I'd probably have more fun playing another Serious Sam or Worms 3D. The other piece of criteria for me is a solid mod community. A mod can take a great game and make it better, and it can also take the most serious of games and make it a riot to play. Mods can expand the replayability of a game tenfold.
I'll use Fallout 3 for an example. Without any DLC, the game leaves you with a level cap of 20, a finite ending (no playing after you "beat" the game), and a handful of little gripes that, although they don't take away from the game completely, are noticeable. Now, before I go any further, I want to say that from the very beginning I've loved Fallout 3, the plain, vanilla, unmodded, un DLC'ed Fallout 3. The game was flat out great as-is, BUT, after fully exploring the game, I felt some things were missing; little things that would have made the game more fun, or the world a bit more believable. Luckily, the Fallout 3 mod community is ripe with everything you could probably think of. Beyond the predictable nude and cheat mods, I've found a whole host of mods that have completely opened things up in the post-apocalyptic game world. I found mods that extend the level cap far beyond 20 on the un-DLC'ed game. I found a mod that let me start out as different character "classes" (bounty hunter, city guard, slave trader, bandit, ect). I found a mod that opens up the game after the "ending" to let you just fool around. I ended up turning my badass slave trader into a pink bikini wearing, motorcycle riding maniac with a gun that shot radioactive exploding flaming dogs. You just can't get that kinda content without mods. I've also seen mods take a game that I would otherwise have little interest in, like The Sims 2, and make it something I would like to play.
The bottom line is that you can use mods to mold a game to your play style and interests; you can take them and make any game uniquely yours, and if you're brave enough to try making some of your own, help to share your vision of the way a game should have been with the rest of the world. Unfortunately, while some developers have greeted modders with open arms, like Bethesda and Valve, others completely ignore the community and sometimes take a harsh stance towards it, like 20th Century Fox. This kind of closed mindedness can lead to stagnant game development and underwhelming and uninspiring games. We wouldn't have blockbuster games like Team Fortress 2 if ID had put the kibosh on the original Team Fortress mod. Would we have Counterstrike if Valve had locked out the modders from Half-Life? If more developers and publishers opened their products up to the mod community, we could have some absolutely amazing games that could inspire some excellent new concepts.