BIG NOTE: all of the following is just more of me *****ing and ranting about this game (because it's a great game, I love it, I hope to see improvements in these areas, and I feel like venting a little bit). So if you're sick of reading what I have to say, press X to skip this post. ER, wait, this might not be one of the skippable ones.
(Also, may contain what may be considered SPOILERS)
Playing co-op makes the game almost half as tough as it normally is (since a death for one person isn't instant start-over most of the time, and you have an intelligent person covering your back--and it all feels awesome, BTW), but then it makes split paths twice as hard as the game normally is, what with having to worry about two people possibly dying and ending the game. Most of the time, this is where covering for your buddy comes into play. And the mechanic works well most of the time. But sometimes it doesn't. The AI's tendency to rush on Insane also makes it much tougher on these VERY linear paths such as the one in the mine in Act III. Of course, the worst part about that one was the spawn glitches with the wretches (My co-op buddy and myself were killed by these *******s spawning right BEHIND us a few times--no crawling from the ceiling, just popping in behind us while in cover). And overall, I have to say the wretches are the toughest enemy in the game. And it feels very much cheap. The Therons are challenging with those torgue bows and tougher noggins... the wretches are just cheap, especially the lambant variety. I wish that the other enemies were the toughest in the game, rather than these annoying distractions that seem like they were meant to get you out of cover for the other enemies to kill you. Same for nemecysts... one hit explosive kills right there. Overall, the game doesn't feel as satisfying just because the most challenging enemies are the cheap explosive variety instead.
It's important, IMO, that a challenging enemy seem worthy of being that challenging. It's why Elites are cool to fight, and Brutes much less so, even though they're often a bit tougher. It's one of the reasons some first person shooters fall flat.
In any case, improvements necessary for the sequel:
Overall-A) More balanced challenge between same path/split path in a co-op game
1) Better AI, that will cover your back more often on split paths ( Act IV, first chapter, as you take on the seeder: Having to deal with the nemecysts, wretches, and Grenadier with Gnasher at the same time in the building felt really cheap. Really, why the hell can't Baird shoot those things down--it's bad enough with a shotgun dude and multiple wretches, it's worse when it's a linear path and he'll eventually rush you, and it's even worse when those stupid nemecysts have an entire ceiling and your side to get to you, pretty much silently as well, and auto-kill you)
2) Less linear design for the split paths. By which I really mean: Don't put me on a road that can only fit one piece of cover on it at a time width wise. i.e., Make it more like the street in Act IV Chapter one, rather than up high or down in the mines. Horrible with rushing AI (Insane).
3) Give us some damn checkpoints on the split paths. The most frustrating part is doing the easy part to get to the hard part over and over and over again. In the game, give us two checkpoints: One is normal, and there's one in the middle of the split path. In the pause screen, below "Load last checkpoint" make a "Load Path-fork checkpoint" which would be the one you go to just before you enter the split path.
(and just to rant again, put your checkpoints AFTER dialogue, not before. I'm sick of seeing the same cutscene or listening to the same dialogue over and over again.)
B) Nothing breaks my suspension of disbelief more than a boss that has 20x as much health as me, but looks only a tad bigger than the rest of the enemies I've been facing. If the boss is going to be tough, make it seem like he deserves to be so, rather than giving him a magical "we need a tough end to this game" field. The other two bosses in this game make sense, and are at least somewhat interesting to fight (just need the right weapon--HoD and Lancer, respectively). The last one is just cheap, and goes back to the old boss crap that should have been purged from FPS game design long, long ago. Halo had the right idea: Boss battles, not boss characters. It screwed up in the sequel, but at least two of said bosses were mostly fair. One actually wasn't that challenging, and it was all the Honor guard (interesting challenge) that made it tough. Fair fight. Another was a little cheap, but not that bad. At least it seemed like it was a slightly intelligent enemy that was causing you all the trouble (hologram decoys/auto-turrets), instead of him just having 20x as much health as you. The last one was crap, but not nearly as bad as Gears' last boss. It might have been better had he taken cover when the flying rats were off, blindfired, had some allies dropped off periodically, etc.