I could name everything my teammates do wrong. But I'd be saying the same thing as everyone else. So I'll name what I do wrong:
Poor decision making - Bad Baron/Dragon calls, allowing myself to repeatedly die when behind, getting overconfident when ahead. Or the worst one, following up on an action that I consciously know is a bad idea.
Poor knowledge of matchups - I keep my champ pool fairly wide, so I can't be banned out of any role. But this results in being put on a comfort pick in an usual situation. Yeah, I can play Hecarim, but I don't know how to play him against Garen, for example.
Failing to work with the team - I agree that it's hard to work with people who listen to me. I consider myself to be a decent shotcaller. But it's soloq, everyone's selfish (including ourselves). If the team won't listen to me, I have to start listening to them. Even if they're making what seem to be bad calls, we're at least playing as a team. If I think Baron is a bad call, but I'm not the jungler, I might as well follow the jungler if I deem it's safe. (If not, I retreat and recommend they do the same).
Little early game impact - I've adopted a fairly safe playstyle. I prefer to let the enemy make the mistake, but that leaves me vulnerable to early game aggression. I'm also a Warwick main (subject to change though).
Overconfidence based on last game - This is a big one. If I just carried a game, I'll pick the same champion, expecting to win. That's basically the biggest noob mistake in the book.