1. Long ass levels - I was thinking about this while playing Flynn Son of Crimson, a game whose levels just drag on and on and on. I like a 2d, side scroller to have levels that are short and sweet. Wasn't there a quote from some Halo developer about keeping a player's interest in five minute intervals? That's about how long I want a side scrolling level to last. Did any stage last longer than five minutes in Mega Man X? Now, if a game has check point saves where I can stop playing a level and come back another time, that's fine. I don't believe Flynn Son of Crimson does this though
2. Spongy enemies - I'm noticing more and more games these days where just basic, low level, enemies can soak up a lot of hits and it just annoys me. You're not a special enemy. You're not a mid boss, or an end of level boss. Just f**king die already! I also hate bosses where you essentially have to go through the same pattern of dodging and attacking for what seems like an eternity as you chip away at the bosses' 10 health bars, or another annoying thing I hate, you must first destroy the boss's "armor" bar before you can start damaging his "health" bar. Ugh. If there is one thing that this and my previous point about long levels do to many games, it's drag down the pacing to a crawl. Then I get bored...and I turn off the game and go watch something on youtube or Netflix instead. Maybe I just have a low attention span
3. Invincibility modes - There's something annoying to me about enemies than can just make themselves invincible whenever they want. I can tolerate it with a boss fight because...it's a boss fight, but beyond that...no, just no.
4. The boss that makes you wait forever before it's vulnerable - You know what I'm talking about. Those bosses that have to do five or six different attack patterns before they FINALLY do the thing that allows you to actually damage them. Even more annoying are the bosses that RANDOMLY go into their vulnerable state so you're just waiting and waiting and waiting for RNG to finally let you actually attack the boss. Or another thing I hate, the boss just exposed its weak point, but you have to do a bunch of tricky platforming to get to it, or kill a bunch of enemies first, but if you took too long...well now you have to wait another five minutes before the boss exposes it again

5. When you mess up a puzzle or something and the game doesn't let you reset. You need to jump on that moving platform to get to the secret item...oh damn, you missed the jump...wait, isn't that platform coming back....seriously? You mean you only have one shot at the jump? FUCK YOU GAME DESIGNER. Yeah, so in scenarios like this you essentially need to start the level over again. Or, if you're lucky, you can just kill yourself...but see, I hate having to kill myself, because so many indie games now have some stupid, punitive measure in place when you die. Like, you lose money, or you lose experience points or some other bullshit.
6. Similar to the above, you see a fork in the road. You go one way...and now the other path closes. So yeah, any cool items that you could have gotten like that health upgrade...opportunity lost. Ugh...like...WHY? WHY THE FUCK DO DESIGNERS DO THIS?
7. Games with tons of collectibles that give you no way to keep track of how many more there are or give you a general idea where the ones you're missing are located. A game that handled this beautifully? Astro Bot Rescue Mission. Also, Valiant Heart the Great War
I guess in general, I just hate whenever it feels like a game is wasting my time