I like Borderlands. I haven't finished it, but each time I played I had a great time, and I think this is mainly due to its simplistic nature and its quirky in-your-face approach.
The game is as addictive as Diablo when it comes to loot dropping-there is always a better gun or set of armor, and it's never that far away. This triggers every wonderful gaming endorphin I've collected over the years and keeps me sated.
I don't want to start slipping into a game review here, so I'll keep things short and classy: I like Borderlands because it's simple.
The story is simple; characters are introduced quickly, the plot falls out of the picture for long periods of the game, and I am left in a desert filled with mad-max road punks looking to take me out. I can get lost in the game because the game allows it. This is all survival in the wasteland really needs to be--finding the next gun, and being able to kill the person or mutant that tries to kill me. Maybe stopping for a breather now and then.
The characters that populate this place are vivid and eccentric--see first boss Nine Toes (Also, he has three balls) and come and go quickly, but remain memorable. It's a funny game, but part of what makes it funny is that the jokes it attempts are spaced far apart, and never come forced. This game never tries to be anything it isn't, and that's why I like it so much.
The core mechanics work and the loot system is powerful, what more do I really need? This is a great way to kill time and play with a friend, because Borderland provides a platform for running and gunning, and just enough juice to get you to the next boss.
Designers can learn a lot from the appeal of a game like this. What Borderlands does is not necessarily brilliantly innovative or outstandly technically perfect--it just does everything in a very solid way, and it never takes itself too seriously.