In my mind (and I see this line of thinking from top players too) there are really on 3 classifications of chars in the game at the moment:
DPS
Tank
Support
DPS are classes you pick to deal damage and kill junk, tanks mitigate damage or jand control engagements, supports enable the first two classes to do what they do better. Mei can't do either of the first two things well, but her kit lets her do the last one pretty okay. You can argue she is somewhere between support and tank on that list since she has high survivability and controls fights real well, but she doesn't have physical presence and still gets bursted.
Some chars get to do a bit of crossover at the expense of lacking other stuff though. Think it would benefit Blizz once more characters are introduced to rethink their classification system with those 3 bases in mind. Hell, split up the supports into "has heal" and "doesn't have heal" categories if they have to. Split up the DPS among the hitscan people and the projectile people, I don't even know. Just anything better.
Just think the current classifications are very misleading to new players or anybody wanting a decent idea on how to build the comp. It's not some game breaking complaint, it's all semantics really, but I do think it's dumb how we invalidated their system so fast.