Hello! I was one of the streamers who took part in Feedback Quest, and I'm here to leave some comments I had while playing.
The opening screen features far too many buttons. I would recommend moving the Achievements, Stats, Relics and Cards buttons to the Character Select screen rather than the opening screen, as it would make more sense to be able to access this information from that screen rather than from the opening screen.
The gameplay can be very long and boring. You're essentially doing the same thing for 15 minutes, but a lot of it is just moving around in dull circles while things automatically attack and kill. Shorter games would be better (maybe only 5 minutes instead of 15).
The cooldown speed of regular attacks is very slow. There is also the fact that on some characters, there is a miss zone between the player character and the attack where enemies won't get harmed. This is should be corrected to remove.
As you get further into stages, the number of enemies and effects causes slowdown, even on higher end PCs. Shorter games would help this, as you wouldn't have as long to collect power-ups and because they would overall reduce the number of enemies you fight.
Only being able to attack to the left or right when enemies come from all sides is not fun. Moving up and down can be stressful and annoying because of this. I recommend making it that you can attack in eight directions depending on which way the player is moving.
It's been commented already, but the shield blocking you from moving is aggravating. Even if the problem is eventually lifted from super fast attacks, it just shouldn't be a problem to begin with.