There are some similarities between Borderlands and Fallout 3/NV, but they're really pretty minor. Fallout has bits of humor mixed in, but it's still supposed to have a realistic feel. The core storylines are serious and contain moral and psychological aspects. Much of Fallout is allowing the player to enter an almost completely open world and letting them define their character in any way they like, with choices that can be unique to each play-through. There is a lot of commentary about human nature.
Borderlands lets you choose a character, but it's still a set character on a set path. There is no real deviation. The character is already established. Borderlands has a semi-open world, but it's segmented and mostly locked. You have to follow the storyline to open up sections. The story has serious moments, but it's very much based on dark humor. It's supposed to be fun more than anything.
Despite both being FPS, the game-play is also very much different. Fallout typically has slower action with more realistic gun-play, where Borderlands is over the top with lots of running and gun spamming. You don't have all the options and equipment like in Fallout. Borderlands 2 doesn't even have health potions, much less the incredible assortment of chems that Fallout does. Fallout is very much about tactics, Borderlands is about tearing stuff up. And that's not even getting into VATS.
These are two of my favorite series, but really, the more I look at them, the less they are alike. Borderlands takes cues from lots of games but in the end it's pretty darn unique.
