One thing that really bothers me is how annoying certain compositions can be to play against. Like Reinhardt/Bastion/X is much harder to counter relative to how brain-dead easy it is to execute. Plus it's not very fun to play as or against, but it's very effective for such a minimal investment of cooperation so tons of people use it. I'd much rather play a losing game against an unusual composition than play a winning game against a Reinhardt/Bastion/X.
I don't know quite how to encapsulate this other issue I have but I feel the optimal playstyle in matches oftentimes ends up being boring or unengaging. For example, I can't count the number of games where I've thought, man, what are we doing wrong we can't break through, only to realize we just need a Reinhardt wall. So I'll switch and we'll push forward and win, but there's nothing particularly fun about just holding up a shield and walking forward blocking the bullet hell while everyone else does stuff. Sometimes I wish the payload had a shield on the front so I could play someone else instead without compromising our ability to win. But it's like, well, this is a prerequisite to actually completing the match objectives so I guess I have to do it.
Same thing happens with healing where your best bet is often to camp out safely until the optimal time to use your ultimate. The benefits of playing offensively is so outweighed by the risks that hiding yourself defensively usually feels like the winning move. Is it effective? Absolutely. Is it fun? Not at all. I've gotten the Play of the Game several times with "clutch" Mercy revives, but all I was doing was hiding behind a corner or box and then pressing Q. Necessary to win those matches sure, but completely uninteresting to actually execute.
I absolutely love all the tactics and strategies associated with side-skirmishes and 1v1s, but those usually are a much smaller piece of the overall gameplay relative to the objective orientated strategies.