visual indicator for reply stacks

[jeisner]

Reply stacks are special because they only contain replies to a single post.
To indicate this, a reply stack should look different from other stacks. Its expanded views should also look different.

How should they look exactly? Let's discuss on this thread.

Here is a possible design suggested earlier (see also followup post) where different kinds of posts (e.g., replies) have different outline colors. Since you can see the edges of the posts in a stack, their colors tell you what kinds of posts are in that stack.
But there may be other good designs too.