I agree about keeping the high-contrast stuff out of the titles column. The titles are so much easier to read since you moved the tags east, I'd prefer that stay that way.
I don't much care for the idea of overlaying the replier avatars, since some avatars will already have overlays due to the thread being stickied or locked. It'd be hard to avoid cases of two overlays on a single avatar, which would be too much.
As for how big a deal this is, I'm torn both ways. The avatar/name mismatch isn't confusing once you get used to it, but it does remain sort of ... jarring, I guess is the word. But I think the loss of thread identity that you mentioned would be even worse, so on the whole, I guess I'm leaning toward "leave it as it is."
As for other ideas ... it occurred to me that it might be possible to reduce the avatar/name mismatch effect by not putting the (boldfaced) name right next to the avatar. That is, instead of "Toby replied 6 hours ago" perhaps it could read something like: "Last post was 6 hours ago by Toby" ... or something like that.
Franz Just to bring up another option: since each item in the list takes up two lines, we could also have to lines on the right side (tags on top, "Adam replied 5s ago" below). Thoughts?
Wow, I just realized that the tags and reply count are centered in the space. That sounds like a possibility. The reply count could go on the bottom line, to the right of the "xxx replied n ago" line, with the small avatar next to the name. Then you could put the name of the OP under the title.
