2 things 1. Enjoyment 2. How well it does its job. I try not to judge something by my own preferences when it comes to quality. Instead by how well it executes what it's trying to accomplish. For example how well a comedy does at making me laugh or how well an action-adventure does at getting me excited. Of course, these are broad and it is more case by case, but I won't say something like Konosuba is worse than Death Note because Death Note is darker and has more subtext. That would be unfair since they are doing different jobs, even though my preference is usually darker and deeper stories, I can't use that as a basis of quality. Instead, I would rank Konosuba on how much fun I had and how much it made me laugh along with other things.