Guns, loot, and more guns.
In Borderlands you play as a member of a mercenary group looking for the "Vault." The Vault apparently has a massive amount of riches. I'm not too sure about the story after that point.
What I am sure of is that there along the way of getting to the Vault there are lots and lots of guns. Sniper rifles, shotguns, handguns, and rocket launchers are all at your disposal. The game does a great job of spreading the guns throughout the game, so you always feel like the next gun is right around the corner.
What good are lots of guns if you don't have anyone or anything to shoot? Borderlands remedies this problem by giving you a vast array of rabid beasts to kill. From a pack of flying "birds," rabid dogs, giant worms, outlaws, amongst others, Borderlands, does a great job of keeping the cannon fodder varied. When killing rabid beats isn't enough, you can kill your teammates.
That brings us to the fact that borderlands should be played with a friend. The split screen coop is alive and well with Borderlands. In fact, I wouldn't recommend playing this game by yourself. One of the most enjoyable parts of the game is that the players don't share money, and they don't have to share inventory items. This means that the chests that are discovered are fair game. Whoever gets there first, gets the loot.
Borderlands graphics aren't beautiful, but they work for this game. The art design is somewhat retro (for lack of a better description).
In the end, Borderlands is a great coop, split screen shooter, (almost) RPG that should not be missed.