As I've said elsewhere, I'm all for a system like this. I do feel that the grading should probably be a little tougher than in the OP and I feel an integral aspect of a system like this is to add tangible ways that the player could improve their performance alongside a letter grade. It doesn't have to be, and shouldn't be, anything extremely specific but just make it clear why they got the grade they did. If a tank takes a lot of unnecessary damage, remind them to use their cooldowns. If a DPS has a DoT that can be maintained indefinitely, remind them of that, etc. Heck, even for healers you can make a note if they're healing too much and encourage them to DPS more often instead of overhealing.