The shooter genre needed this. Elements of role-playing games have been creeping in all over FPS games in the past few years, but in Borderlands it's a wholesale hybridisation. Not, I should point out, in terms of choices, story and consequences - that remains with the likes of S.T.A.L.K.E.R. - but with loot, levels, stats, skills and fiddling about in your inventory to max out your character. Gearbox says it's created a role-playing shooter, an RPS (which sounds strangely familiar to my ears), and that means you'll be playing a shooter that feels a lot like, well, like an MMO.
The common touchstone for talking about Borderlands' RPG influence has been Diablo, but I think until the third Diablo game comes out it's probably just as - if not more - valid to mention that Borderlands exhibits a large number of MMO-like characteristics. This feeling is at its strongest in the opening areas of the game, where you're picking up missions, running back and forth across small areas of the map, collecting loot and killing low-level punks and mutant dog-lizard things. Just like most mainstream MMOs, Borderlands takes time to hit its stride, and you're hours in before you really start to appreciate the approach Gearbox has taken. That's not to say it's not an entertaining game from the outset - because it is - just that it really does take time to unload all its tricks and have you revel in them.
There are a couple of reasons for this slow build. One is that you're probably going to play the first bit of the game on your own. And that's fine: single-player in Borderlands is entirely valid, and fun. But the sense remains that there's something missing. This is filled when you start playing co-op, because one of the most obvious mechanisms in the game suddenly makes sense.
That's the "second wind", in which you're granted some time to try and get a kill when you're reduced to zero health. Murder a bad dude and you get back to your feet and continue to fight. However, when you're playing solo this often means you're dying in some corner you've retreated to, with nothing to shoot, or no hope of killing what's in front of you. In co-op, it's the window in which friends rush to your aid and pull you to your feet. It suddenly becomes a sensible idea that boosts the experience for everyone involved. All signs point to co-op.
Co-op is arguably how Borderlands is meant to be played. While the quests in the single-player are adequate to the task of keeping you occupied and entertained, the ludicrous ramping of enemy numbers and power when friends join makes the experience far, far more chaotic, and therefore more entertaining. Being able to keep each other going in that Gears-of-War-buddy-system way means that large fights can roll on without you have to beat a hasty retreat from heavy resistance. The way the game scales, the character types overlapping, means that any number suits the game. Two or three players is just fine, no matter what the task at hand. The soldier character can even act as a healer, shooting health into his team-mates, while plenty of other passive effects from each of the characters boosts the group in various ways.
With four players, it's a riot, and they can drop in and out as you go. Campaigns are set up so you can get three other people to come in and join your particular quest arc. As the host it's your storyline people will enter, but they still benefit from being there: levels, weapons, and missions collected in the online game transfer back to everyone's single-player game, with anything that's out of your level band simply inaccessible until you've got to a higher level. I suspect playing with strangers might be a bit of a task, mind you, as there's no loot binding, and anyone can pick up anything. So watch out for loot-hoovers.
It's worth noting that playing a high-level game with a low-level chum as sidekick means that you power-level the low-level character. They will, of course, get hammered by the high-level enemies, no matter how much you tweak and add shield capacity to that newbie inventory, but it's a very speedy way to get someone to catch up, and to get their inventory full of new kit.