Another big problem in high mmr ranked (I'm talking 5-6k+) is that sometimes when you have a bad game, people report you for playing bad which is not what the report system is meant for. Reason why I pointed out high mmr is because this is more common in 5k and above since everyone thinks they're the best and tend to report people for playing bad instead of for behavioural issues.

The worst thing is when you're the highest mmr and the rest of your team is way below your mmr and they flame you when the team loses because 'you failed to carry and you're an account buyer'.

People don't seem to understand that you can't win all games and not all games are determined solely by skill. Even if it was, people don't seem to understand that some players play badly once in a while, and it happens to everyone. Picks play a very important part in games and I guess that ties into your first point as well. The picking is very imbalanced right now and I always hated it when I get last pick cheesed by a really skilled high mmr Meepo back when it was popular or something similar.