Hey, look! I just played Borderlands again! But wait! If I continued to play right now, I would get to the DLC! Which I did. They all had their own pace and personality. These are larger bits, sort of like mini-expansions, more along the lines of Dawnguard or Dragonborn than your usual map pack or me-too type quests. I played through half of these before I got really going in the main quest, so I'll try to recall this as straight as I can. I did that in order to level up ahead of the enemies encountered, so everything should be a little easier. And was it ever! It was really nice to have started a new character, since it is enjoyable to have enemies that aren't always at least a level above you. I will also go over these in order of story, not order played (which differs).
The first is The Zombie Island of Dr. Ned. This is a play on the doctor from the main quest (Dr. Zed), who is not a real doctor, and this "brother" or what ever is not any more legit. One of the gun manufacturers, Jakobs ("If it took more than one shot, you weren't using a Jakobs!"), had some sort of historic cove thing, and their local doctor, Dr. Ned, went and messed that up in short order. There are zombies everywhere, as are Halloween decorations. The only notable thing I liked was it brought more color into the game, even if it is a dull gray green.
Mad Moxxi's Underdome Riot is the one I least enjoyed. In fact, I haven't played all the way though it, nor do I want to. It is basically an extended PvE arena fight. In order to spice things up after a while, certain effects are applied, like certain types of guns are really effective, and others are severely gimped. There are several waves of enemies, normal gun enemies, psychos, bosses and so forth. I stopped when it became apparent that I wasn't having any fun. Psychos plus only rocket launchers equals death by friendly fire; literally, because my only launcher was a Maliwan fire one.
The third was The Secret Armory of General Knoxx. I probably had more fun with this than the others. The idea about this is that after you take care of the mercenary general that comes after you, they send in the big guns. The Atlas Corporation really wants to take over the place. You get transported to what apparently was the middle of some sea (that has since been drained somehow), and start doing stuff. Soon, you will get the attention of General Knoxx. I love this guy; it seems like he is tired of what he is doing, and he just wants you to shoot him in the head, and is quite up front about it. He's a pretty funny guy. Then you get to blow up some stuff.
Finally, there's Claptrap's New Robot Revolution. With Atlas out of the way, another manufacturer, Hyperion, has filled in the void. At some point, the Interplanetary Ninja Assassin Claptrap decides that it wants to liberate all the other Claptraps from their puny human overlords, and styles himself after some Communist revolutionaries and dictators. Soon, you will encounter claptrap-ized versions of regular enemies and animals, then go defeat resurrected claptrap-ized bosses that you've defeated before. It gets somewhat ridiculous in places, in the funny way.
I still recommend just getting the sequel though. I have most of the DLC for that too, and I've already played through it once. I will be doing that again, so I guess that's something to look forward to.