Things overwatch did wrong:

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>every hero can headshot
When headshoting is the default, then everyone shoots at head height. Even a healer like Lucio. It just makes the highest potential damage you can do to be more difficult to be achieved. That's not something to slap on every character.
>every hero is a sniper
In tf2 all classes have incredibly mediocre damage at long range aside from the sniper, so you could actually analyse your close surroundings and feel safe, that is not the same for overwatch. A Lucio shooting at head height, can drop half of your health in an instant from 10 km away.
>League of lolz ultimate mechanics
Most retarded game design that has ever existed. "Some little kids can't feel like a super hero? Let's just give them a button to allow them to wipe an entire team"
>League of lolz infinite character adding
In tf2 tournaments both sides play the same characters, there's no problem. Not the same for Overwatch. They'll have to destroy the skill potential of characters to make it balanced; just look at tf2 rocketjumps vs pharah press a button to fly slowly.
>No team colors
Slap an ugly as fuck red silhuete on enemies. Genius. That, combined with the fact that everyone shoots each other from afar maximizes the shittines of your visibility.
>stale meta, QP and ranked are the same
There's no such thing as casual play in Overwatch. You join to play a meta and to win. And if you don't, you just die over and over and have no fun. Overwatch successfully brought the worst of league of legends into a tf2 style fps.

The original tf2 devs knew that in order too make a comfy gameplay, you needed comfy rules of engagement. In Overwatch, with over 20 characters, with almost every character being able to headshot, and every character having a team wipe ultimate, that becomes impossible. So they try to compensate this by crippling the team size to a limit of 6.
I'm not a hater, but I don't think people will becoming back for Overwatch after 10 years.