Borderlands' biggest fault was being launched at the same time with so many other good games. It fights with L4D2 on the shooter side, Torchlight on the Diablo side and soon Dragon Age will enter the fray, fighting for gamers attention.
I stopped doing any sub-quests in Borderlands because I want to finish it fast, and honestly, the game is easy enough without me being only 2 levels above all enemies. But the truth is, the gameplay has started to turn a bit stale for me, and I want to complete it before my patience runs dry.
I guess my biggest gripe with Borderlands lands is that I can easily do some things, but at the same time I can fail miserably at others. In my opinion, the game doesn't have any huge flaw, but its plethora of small nitpicks end up wearing me.
In the end, it was the world and the characters - not the gameplay - which captivate me. I want to stay on Pandora just a little longer before saying goodbye.