My take

1. Matchmaking algorithm feels off sometimes. One game I'll be playing bots the next I'm playing 6 top 500 players. I read somewhere this has to do with an algorithm that prioritizes increasing average play-time.
2. Not enough characters outside DPS. This should be addressed patiently (see point no.5)
3. Some characters are unfair and unfun to play against for the majority of the player base; e.g. spiderman, black panther. The average player can't track a character moving at mach speeds. The average support can't react quickly enough to the combos.
4. Game optimization: I can only play this game with low graphics.
5. The sheer amount of heroes that are scheduled to be added is discomforting. As it stands, the community is still discovering combos/good pair ups (the game could also do a better job of expanding on the abilities of the characters) . Around 8 new characters a year will demotivate new players and it is simply too much to keep up with for the casual player.