This isn't actually a bad idea. Although I think Theymos was more in support of retaliatory feedbacks as a form of replying rather than being able to actually reply directly... And as the others have said, it would make the page look a bit messy.
Maybe a small link with the response, similar to how reference is currently used? I think grouping feedbacks left by a user and having an expand link to limit the number of lines viewable by default in the comments would also clean up the look a bit.
Hmm potentially. I've been thinking something similar with merit too to have a way to add a short comment that isn't really worth posting but is worth leavingfor the user as maybe to tell them why they earnt that merit.
Something similar to that might be a good idea for both merits and trust in order to allow users to give their side of the story while someone is reviewing their trust history (although a link where it is hidden is going to mean that a lot of the noobs that call people out on this sort of thing will probably miss it).