This is my opinion, not fact.
The short version: I don't like getting one-shot.
Overwatch is a modern version of instagib for me. I mainly played mercy, and I always die in one shot / combo.
I know that not every hero can one-shot, but I always have the feeling that these deaths feel the worst. Forgot about the Widowmaker for a split second? You are dead.
Even as a Tank / DPS Hero, I don't like these mechanics. Being killed really quickly is just not fun. I like longer fights where you have to think about Munition and Cooldowns, not about being killed instantly.
- Roadhog - can still one-shot, even after his nerf. Get hooked, and there is a high percentage that you are dead. He can even hook you in a different position, even though Blizzard said they fixed that.
- Widowmaker - Hold the Right-Mouse-Button, shoot and kill half of the Heroes with a precise Headshot instantly.
- Hanzo - The same as widowmaker, but can kill multiple people very quickly with his Arrow-Spam E Ability.
- McCree - Either he gives you a Headshot, or he stuns you, fans the hammer and rolls out. Barely any chance to counter when you are stunned.
- Tracer - Can blink every few seconds and kills you either with her bomb instantly or with a few headshots in a matter of miliseconds.
- Junkrat - His Riptire ultimate ability's only weakness is height, so if you are mercy and don't have your ultimate then beg that your teammates shoot the tire.
- Doomfist - His ultimate, if well placed, can kill an entire team at once.
- Reinhardt - Even though this is rare, if he pins you during a charge you are dead.
I like interesting and complex gameplay, but being one-shot by someone who is just aiming and shooting like Widowmaker is just not fun.
This wasn't a problem earlier. I'm currently around lv450, and during my first ~200 levels I never had any problems with any Heroes.
But now: Every Quickplay-Game has atleast one Sniper (Widowmaker/Hanzo) and a Tracer. And they are playing really good, as if their life depends on it.
Why not play Competitive if you are trying to win that much?