Variety and adaptability are what characterise Borderlands' action. With so very, very many different weapons, so many customisation and playstyle options, the idea is that shooting things never gets dull. There's always something new to try, some new configuration, and with the added variable of other human players, it's easy to see how this could be hugely entertaining. Much in the way that Halo's unpredictable enemy behaviour made it almost inexhaustibly enjoyable in co-op, Borderlands' randomly generated weapons, enormous world and vast amounts of collectible swag should make Hitting Things With Guns a newly revitalized experience.
Halo is definitely one comparison; Borderlands seems to be shooting for the same 'ten seconds of fun' formula, and a similar mix of vehicle-based and on-foot action. The other resemblance is, oddly enough, Diablo.