For me, it seems to be the lack of effort or care that teammates seem to have per game.
Sure, it’s just a game, but I would like a little effort instead of instantly leaving the second we lose a point or push, or switching to a hero that does not provide any real help in succeeding when those two factors come into play.
DEFEAT
This game is too casual, and there is no real incentive to make people want to win. Even competitive is full of leavers and people who don’t care. For a game that is so team oriented, it really socks that half the players don’t seem to understand this
Brig quite easily is killing my mood to play.
2nd place is the number of times I’m backfilling games that are 30 seconds - being at the defeat screen when I load in.
Not only being robbed of a rightly-deserved victory, but colossal levels of both ignorance and elitism from people both in the game and even on the forums. That really takes away the motivation needed to play.
Not being able to play my favorite character at all is getting a bit frustrating. Like, I’m fine with counters and all that, but no character should 100% force a hero swap.
I’ve been just playing Brigitte myself since it’s the only way to win (77% win rate in QP atm, it’s so ridic). But I really do miss being able to play Tracer. =/
Extremely long queue times (I end up waiting longer to play than I actually play, without having been avoided or without avoiding anyone)
Toxicity and sexism the moment I try to play someone new
Very few heroes are actually viable
Very few heroes to even choose from
Matches feel one sided (either we win by steam rolling or we get steam rolled)
Lack of attention to modes like FFA which I find more fun lately
No real feeling of progression or achievement
If I’ve said once, I’ve said it 1000 times.
Teammates are my biggest counter.
One of my favorite things in the world to do at the end of a long day was to drink too much and play overwatch. I quit drinking a few weeks ago and it just don’t feel the desire to play. Weird, but true.
That isn’t 20 characters. What the heck
What kills my mood is when I finally get a chance to play something besides a healer…
…and the healer doesn’t heal 
My lack of motivation to learn Hanzo.
I put hours into McCree only for a Shimada to outshine him in everything, kinda sucks, feels like that time was wasted now.
But jokes on Hanzo, McCree still wins in style points.
People not choosing healers when I just for once want to play DPS. 
Simple Geometry. mate.
it’s magic
All my favorite heroes getting nerfs. Kinda demotivating.
Teammates who get overconfident, every time this happens we lose because they think they can do anything and will still win because we did great the first round and has lead to many a lost game
Honestly what kills me, is that there are obvious hero’s that will enable the win, and people…refuse to play them.
Its comical how contrarian people are.
Too much Hanzo/Widow.
Maybe it’s just because I main Brig but I don’t find her that oppressive to play against when I Rein or Hog or Zen or other heroes. Whenever I Rein the enemy team goes anti-tank comps to punish my playstyle, Brigitte is no worse than the Reapers and Junkrats who try to walk right through my shield to kill me who I’ve played against a million times before.
But Hanzo/Widow. Ew. Most of the people who play them have truly horrendous positioning because they know almost no heroes can punish them for it because of enemy heroes’ damage falloff or lack of a OHK. Honestly most Hanzo/Widow teams are borderline impossible to dive even when they don’t have a Brig because the moment you dive them, they grapple/dash away and melt one of your tanks or kill your backline from halfway across the map.
My last game was a Widow/Hanzo vs Widow/Hanzo sniper duel. I was on Rein. I almost fell asleep in the middle of the game. We won. But it wasn’t satisfying. It was just boring.
In competitive:
Solo queue is too toxic and random while groups are hard to form and then penalized by the match maker.
In QP:
It is only really worth playing with my friends because we try to win, but they aren’t always on. If only I was part of something bigger like a clan where I could find like minded individuals to play with.
FFA:
Is fun but it loses its appeal after a few matches.