Whenever a person concedes, give them an option to describe why they conceded. It could have some standard choices like "I was about to lose anyway" or "opponent was too slow", etc. I would like an option for Brawl that lets me say "I quit because the opponent was using Alchemy/cards banned in paper multiplayer EDH" . You could also add a text box where a person could say something like "I hate Mythweaver Poq" or "Primeval Titan is annoying"

With time this could give you some feedback on what players enjoy/don't enjoy about the platform and your choices of format styles and card availability.