I found Borderlands' concept to be fine, but the execution to be way off.
A lot of enemies were just bullet sponges that required nothing more complicated than circle-strafing and shooting. The actual loot mechanic was subverted by having high level weapons near spawn points in cities, while bosses and enemies would drop kind of shitty weapons compared to what you could get by just wandering around town a bit. There was almost no level design to speak of, either it was just corridors of caves, open desert with an occasional corridor in to an arena-type area with exploding barrels, or mountains that were walled off in to basically corridors. The PC port was embarrassingly bad, and moreover,
they straight up lied about it. You had to scroll with Pg Up/Pg Dn buttons, change the text size to see the full descriptions for every weapon, play through
Gamespy for some unexplainable reason that likely has everything to do with Randy Pitchford's infamous hate boner for Steam that he eventually got over but not in time to actually help the game any. The characters you chose were stocks that had no actual personality and very little gameplay flavor - there was almost no advantage to having a crew of four people with different classes since the game did not take advantage of their skills (though this is a problem that permeates almost every part of Borderlands, it feels like gameplay systems were designed separate from everything else and then pieces of the game were just stuck together with glue after having been finished separately). The enemy design was basically "This is a wolf. This is a person. ITERATE ON THESE FOREVER."
The more I think about it, the more I realize I did not actually like Borderlands that much! And I honestly have no faith that Gearbox will fix everything wrong with it because of all the praise they got for the first game.
I will say that one part of Borderlands felt truly great and like it was building to something. When a buddy and I went in to a city for the first time and were attacked by men in armor with proper munitions and it was challenging and fun. The city had actual level design, interesting new enemies, and a compelling story premise. I thought it would be a turning point for the game, but nothing for the rest of the game was that interesting again.
Oh, and I thought the game was really unfunny, including and especially the claptraps.