I have found message list indentation in threaded view quite confusing.
When I see:

Some single mail
  v  First mail in the thread
          Second mail in the thread

It seems to me "at first sight" that the second message is the follow-up to
the first. In think the arrow should be in it's own space, and all
"primary" messages should have the same indentation, like this:

     Some single mail
  V  First mail in the thread
          Second mail in the thread