I finally got my wife to try the first Borderlands with me tonight, and I was explaining to her how many clones there are out there of randomized loot/grind for upgrade style games(not sure if there's an actual term for that style of play). Look at how many Diablo style clones there are, or even take it a stretch further and look at MMO's. There are hundreds and hundreds of fantasy/point and click/RPG style games where it's essentially killing hordes of enemies in search for basic loot, but none of those are shooters, or even remotely close to the style of Borderlands.
The only real close game that I can think of is Hellgate: London, and that game died a long long time ago. Sure there's a few that are MILDLY similar, like Bulletstorm, or even to a stretch Destiny. But Borderlands is/was extremely successful, and is the only console game of its style that has splitscreen. It saddens me a little bit that once we play through all of the Borderlands, we won't be able to share the same progression style of play, or carrot on a stick, that we do with Borderlands. My wife has no interest in playing games like Diablo, or WoW, or even Destiny, but she really likes Borderlands.
I just am somewhat perplexed as to why with how clone-happy the gaming industry is, there simply hasn't been any attempts to truly try to replicate the style/success of Borderlands.