A huge open world(s),
More loot types like chest, head, boots, legs....
More skills. 
Heavy weapons!!!
More waypoints.
More vehicles.
More stuff to find.
How about dungeons?

Class specific story lines. I'd love a better reason to try out the other classes. 

Borderlands won me over and I can't wait for 2!

I'd like to see some customizable weapons and a better inventory. A locker would be nice to keep items you can't carry. I like some kind of limit on how much you can carry but I was always dropping awesome guns in Borderlands... it felt like a parent picking between which kid to leave in the desert and which one to keep. Ok maybe it's not like that, but it's like that...jk

btw Where the F is the CLAPTRAP action figure? It was supposed to be out last August!

Scale back the whole "bazillion" guns thing. It was a great bullet point for the original game but way way way too often would you come across guns that were far too similar.  
 
Elemental effects should be handled more practically rather than a "this only works in this situation" For example, as it stands now, Fire>skin, Corrosive>Armor, Electric> Shield. If i hit shirtless guy with a corrosive weapon there should be some type of effect. Even if it's not as effective as fire it should still do something (maybe slow them down?) because after all, it seems kinda ridiculous to think i can hit someone who's topless with a bullet thats the equivalent of acid and yet they feel no affects

Scooter's franchise expansion.

Because four wheels is always better....than zero wheels.

How about a better story? More variety in missions, weapon customization and better difficulty options.

Make the world more interesting and a good ending.

I'd like a single player that was fun. I tried the first game, really didn't like it and everyone tells me "Oh, that's because you weren't doing multiplayer."

That should not be a thing.

How about having some semblance of a story in it, or enemy AI or a game world that doesn't feel like the developer stopped making it once the layout was complete.

• Proper shooting mechanics - the game goes with either bodyshot spam or headshot spam but it's all spam all the time.
• Different artstyle - while, at first, I really liked the game's look it does become distracting and disconnecting when it comes to weapons - they all tend to look the same - as everything does. It's like Gears but instead of browns and dirt it's pastels and cartoon lines.
• More enemy variety.
• A real story.
• No more CRAPTRAP.

Fix the framerare
Add some kind of look acceleration so the shooting doesn't feel like ass
Decrease the reliance on loading screens
Make it more viable as a singleplayer game
 
More varied quests/locations/guns are a given and have already been said 20 billion times, so I thought I'd go for some of the less obvious ones.

Instead of just collecting guns, allow players to collect gun parts so that we can make custom guns based on the parts you get.  Their procedural gun creation engine should be able to cater for this.

better upgrade system/more rpglike skills/stats

I would love it if this time around there were some decent end game bosses / content. The last boss in the original game was a major letdown.