This may have come up already but i think it's a fresh idea.
I try to leave feedback for the majority of players i play. Especially while i was premium, that was quite a large number of people.
I propose that in the list of finished games, next to each players username, there is a little check which tells you which players you have left feedback for. Sometimes you play the same person twice and have already left feedback without realising and the little notifier would let you know.
I imagine it would be a little green tick to say you have left positive for that person, grey for neutral and red for negative. When you click on the tick, it takes you to the feedback you have left for that person.
I would find this useful, what do other people think
priority 3