I created a new forum sidebar in which we group new replies by post instead of just listing it by post title. The advantage is that you get a better picture how old the posts are and that there is less to read since it does not repeat the post title for every reply.
The disadvantage is that you have it a bit harder to find the latest replies across several posts since they will be further down in the list.
There is an open question if we should limit the list. Currently I list the latest 20 posts and their replies.
Could you change the color of the reply from blue to a different color? That might make it easier to see the difference of the title and replies.
Looks great.. at least now the whole list can not be quick-washed by a new thread with 10 replies.
I have no problem making the difference between the initial message and the replies, with the tab offset.
If you want to reduce the size of the whole thing you might only show the last reply instead of the whole list.
I do not mind it being long since we have the space at the last lost of the sidebar. I might rather limit the maximum age to 1 months or so.