Joystiq has put together two separate previews of Gearbox's Borderlands - the first focuses on the single player game, while the other focuses on the online co-op component. From the former:
Perhaps it's shameful to admit, but killing enemies and leveling up is always fun. Being able to level up in a FPS is what brings many players back to the multiplayer lobbies of Call of Duty and Resistance 2. The rewards for leveling up were immediately apparent, as our HP increased dramatically with every level. The game tracks weapons use, and in turn, skill and efficiency are boosted for a particular weapon the more it's used. Depending on the class you choose, you'll be able to grow your character through a skill tree, each with drastically different bonuses. We were able to eventually unlock the ability to summon a turret, which also conveniently acts like a shield. With the promise of additional abilities down the line, it's clear our characters will become quite powerful ... if only it didn't take so long to get there.
Grinding is clearly going to be an important part of the Borderlands experience -- and MMO players will find no problem with that. In many ways, Borderlands' story feels like an offline MMO: talk to NPC, acquire quest, kill targeted enemies, win loot, get XP, and repeat. We found ourselves spending a lot of time partaking in the game's various side quests: finding stolen food, exploring a cave, and clearing a creature cache. Like many other MMOs, there was little draw to actually partaking in these quests other than to gain XP and level up. Many of these missions are presented with nary a line of spoken dialogue, appearing simply in a text window that pops up when speaking with certain NPCs.