I personally wouldn't mind a medal reward system for content, though I'd definitely want it to have a mix of personal and group grading.

For example in group grading, completing the content could have a base reward just for completing the content but then have it augmented by collective group effort, such as:
Abilities stunned
Abilities silenced
Avoidable attacks evaded
Stack markers stacked
Damage mitigated (doesn't include self)
Players KO'd
Time completed
Etc.

For personal, it should rely entirely on your job:
If you have positionals, how often you hit them (the tank could make this harder, for sure, but you also have True North to help you with situations where you miss positionals).
If you have a buff you need to maintain, how often you lost it.
If you have mitigation abilities, damage mitigated (for self as mitigating others is listed under group)
Etc.

For general personal
Enemies killed
Enemies killed that you assisted in (like in PvP)
Damage contributed to the content
For healers, health restored could be added to their damage grading (health restored above max shouldn't be graded)

Like many things in the game, it can be abused by anyone looking to be jerks (not unlike currently running content and having "that guy" looking to ruin someone's day in the game) but I think this would be a neat addition.

I could also see it being segmented by (and probably should for personal) based on what the content is being attempted. Any dungeons pre-50 should definitely not have the same personal grading as a 71-80 dungeon, for example.