To start things off here, the things I find annoying are 1: reflecting bullets, 2: being able to blow yourself up and barrels and why they are annoying.
Tell me your thoughts on annoying things in Borderlands 2 and if they should see a future change in the very possible Borderlands 3 (which was hinted, not confirmed I guess).
(EDIT: changed the title from annoying mechanics to annoying things to be more general in terms of topic :))
Not really a game mechanic but something I find a little bit annoying.
I’m a PC player and GBX made great improvements in the design of the GUI in BL2, to make it mouse’n’keyboard friendly, compared to BL1. However there’s still a way to go.
If you have drag’n’drop somewhere, you should have it everywhere. You can’t use it for the bank or stash.
Clicking on an item in a frame/panel (e.g. skill trees, inventory) should give you “focus” on it. You shouldn’t have to select the frame and then select the item.
If you have mouse select somewhere, you should have it everywhere. You can’t select game difficulty with the mouse.
I’m probably being picky because I spent my career in IT developing business systems and inconsistencies like this wouldn’t have got past my QA team 
Aside from the well-documented assortment of glitches, I have no complaints. Even the glitches add character.
So, nah, I’ve got nothing to gripe about.
Oh and… more guns would be cool…
Skills that have had poor scaling for what seems like forever (Helios, Cloud Kill, Blight Phoenix, half the skills in the Gunzerker Brawn tree). This really bothers me in any game that uses skill trees. My feeling is that if you’re going to have all sorts of choices then all of them should be worth choosing in late-game scenarios based on different playstyles and not just a place to dump points so you can get to the next tier.
Skills that are basically pointless / filler because they’re multiplicative instead of additive or they get factored into the damage formula in the worst possible place. Same gripe as above.
The Siren being the only class that can reliably heal other players. I like having choices, even though I would probably still choose the Siren.
Salvador is a lot taller than I remembered!
My biggest complaint is kind of hard to describe. So you’re in a level and you’re trying to run somewhere. Problem is, you either hook left or right and suddenly can’t move. There’s either a small little overhang or the area is textured weird. You then have to crouch and move in a specific direction to become unstuck. The worst offender is the first set of stairs in Washburne Refinery.
Yeah, that’s pretty annoying and why I never simply walk/run anywhere. I always jump everywhere I go.
Bounce bounce bounce…
Nothing annoying to me about the game mechanics, I actually like a lot of the glitches in this game. But Claptrap… Claptrap.
Reminds me of level up my Sneaking skill in Oblivion 
. But on topic enemy that glitch out of the current area and you can’t kill them in order to advance in that area (i.e Digistruct Peak and Hyperion Slaughter Dome- I know these are optional areas but still…).
When enemies spawn with certain e-tech weapons and you are completely unprepared for what is about to happen.
Hyperion Predators with E-tech rocket launchers (2 shotting the 4 player squad on Hero’s Pass)
Bandits with spikers (FFYL infinite loop due to stacked explosions)
Hell i even got a SGT loader with TWO Hyperion e-tech splatguns.
It would be nice if they dropped the weapons they were using…
That was a thing in Borderlands 1, but sadly the mechanic its self did not make it to Borderlands 2.
I hate the Arms Dealer mission and all missions of this type. I always have to put one car in the middle and travel through catch a ride machines and the fast travels in the Highlands. Argh!
Time attack missions, rabids, enemies with bullet reflection, the Cobra, the Stink Pot, skills that do not scale well, the Greed, the Judge, and a few other things.
The Judge, man. Why they made it?
The fixed Tediore barrel is the reason why it sucks. If it had a Jakobs barrel–hell any other manufacturer it would be better off.
The Stink Pot is the true abomination that needs to be set on fire and ground into dust. Why on earth the people that made Scarlett’s DLC thought a Jakobs weapon relying on DoT damage to kill anything is a good idea is beyond me but it exists. Does not help it is unaffected by a lot of buffs such as Reaper makes it absolute garbage. WHY THE HELL CAN IT NOT CRIT?!
The Cobra would be alright if it was actually explosive and not a non-elemental weapon with an explosion and also not harder to find than a Jackelope.
Greed would be alright if it could spawn with a scope and perhaps having a different red text effect.
I like the Greed, but the Stink Pot…
And the Cobra, well, when I find one I tell you about it
Also, I wish that the Judge had the Hyperion barrel. It fits in the Assassin background and adds a little more critical damage.