The guys over at College News have penned an article that lists five gameplay elements that they think will make Borderlands a better game than Fallout 3. One of five:
Shooting stuff is always better with a friend. Or three. I love co-op multiplayer as much as the next guy, but there needs to be some depth to go with it. It's hard to say if Borderlands is going to have any.
As much fun as Fallout 3 was, it would have been even better if you could have had a buddy with you romping through the desert. How that could be effectively accomplished is unknown, but it still would have been pretty awesome.
Borderlands doesn't give you just one friend to complete quests, race vehicles and slay indigenous creatures with. It gives you three.
The Hunter class is a master of long range damage, the Berserker uses powerful melee attacks and can be your team's tank and Sirens utilize stealth for quick, powerful attacks. Each class also has various support skills depending on where you allocate skill points, and it should be a load of fun custom tailoring your team for the various obstacles you'll face in the game.
For instance, the Beserker can charge in, hold the enemies' attention and absorb hits, the Hunter and Siren deal damage and the Soldier can use his prowess as a medic to keep the team alive. In this regard, it seems like Borderlands implements the Holy Trinity (Tank, DPS and Heals) of multiplayer RPG's, and it should be interesting to see how everything works out online.