Since basically the dawn of time there have been two major annoyances I have had with almost every first-person shooter game:
Firstly, reloading. I hate reloading as a game mechanic.
I understand that having to reload adds a certain level of realism and challenge to the game. The problem is that when a game mechanic is more frustrating than challenging, it becomes questionable whether it's a good game mechanic at all.
You wouldn't believe how many times I have died because the playable character runs out of bullets and starts reloading right during a crucial moment. You wouldn't believe how many times this has happened right when just one or two more shots would have killed the enemy. And of course reloading takes forever in the heat of battle.
And yes, I do reload manually whenever I can. In fact, I reload as much as the situation allows me. But that doesn't help in a very difficult fight where there simply aren't any pauses that would allow reloading. Basically if you reload, you die. And of course you run out of bullets at the worst possible moment, and die.
Dying when just one or two shots would have killed the enemy, and you start reloading, is frustrating to no end.
Secondly, you get constantly stuck on the scenery in fights.
The first-person perspective adds tons of immersion to the game, which is why it's such an ubiquitous and popular format. However, it has a huge disadvantage: You can't see what's at your sides or behind you.
Difficult firefights often involve lots of strafing and backing to avoid the enemy. And you need to constantly aim at the enemy if you want to shoot at them. But since you can't see what's at your sides or behind you, you will invariably get stuck in scenery that's out of your field of view, which can be highly annoying, especially in difficult firefights where you need to be constantly mobile and run and parry a lot.