I'm really looking forward to that game, but I hope they fix some of the problems from the original. I won't get into the story (I actually do below

), but I do hope they fix some of the problems.
I've mentioned this in other places, but I think the largest problem is that the DLC felt like an afterthought in regard to how it integrated in with the game. The biggest problem for me is that there were no adjustments made to talents when they released a new expansion. Now, not all of them
required it, but Knoxx provided 11 extra levels (50 to 61) and Claptrap's release included a patch (for
all players) that provided 8 extra levels (61 to 69).
The issue you ran into here is that talents are usually designed to limit you to certain amounts of power, but that doesn't happen when you can literally grab almost all of the good talents. Talent choice becomes nearly ineffective.
Then there was also the fact that they didn't include the fast travel hubs in the DLC, which usually led to a bit of an annoyance having to run around and kill enemies (Ned, Claptrap) or drive around and try and avoid enemies (Knoxx). They made our lives a lot better by providing awesome quests like
Braaaaaaaaaains, which gave us a reason to kill all of those mobs.
Fortunately, PC modders added a fast travel hub for all of the DLC, which was a bit better. That means nothing for those silly console gamers though.
I'm not saying Borderlands was bad mind you. I just think it's overrated, and I personally couldn't get into it.
I don't think anyone can tell you that the Borderlands quests are good with a straight face. But honestly... I didn't really care about the quests that much. That doesn't mean that I
didn't do them, but rather I was giving them the "Wow Treatment" where you just do quests but don't really care about them. I think that's partly because the story wasn't good (or really... it was very non-existent) in the original game.
But I think I said something important in there... the story was probably the worst part in the original game. Gearbox has stated a few times that they are trying to remedy that problem in Borderlands 2. I've been reading the write-ups on IGN about the characters (they've done Maya the Siren and Axton the Commando), and they've mentioned a couple times that they're even trying to flesh the character's backgrounds out.
Out of all of the quest sets, I would say that Knoxx had the best.