As much fun as I am having playing through Borderlands 2 in Co-op, it always starts off more than a little bit frustrating since my buddy starts playing ahead of me when I'm offline and I always find myself playing with someone 5-8 levels above me. Every session where happens starts out boring as hell: my friend, who is always more of a “rush through and kill” to my “explore every nook and cranny” tends to kill every enemy in the area with extreme ease due to our level difference, leaving me with little to do but follow behind, looking around for loot boxes that he missed. At some point, he then pushes a variety of high-powered weapons and items he found while playing earlier on me. Inevitably these are better than the weapons that I have actually found, which bums me out because discovery is such a huge part of these kind of games and just having stuff given to you kind of sucks some of the fun out of it.
None of these things are unique to Borderlands: in fact, they are issues inherent to all multiplayer RPGs. The first hour or so of multiplayer always seems to go like this, but luckily, Borderlands does a good job of balancing things out quickly. Levels tend to come fast when I'm playing co-op: I'm not sure if its because we're running through missions faster because of the level discrepancy or what, but before long, the enemies have toughened up and our level gap has closed to two or three. Soon after, item drops seem to get better and I can replace some of the high-end gear I was given earlier. I think it may be impossible to completely remove the frustration of mismatched co-op, but I have to admit that Borderlands 2 does a damn fine job of mitigating it quickly.