It strikes me that 90% of complaints about design seem to be "Make this minor element stand out more on the page."
So, it is nice to make an opposite request:
The horizontal lines between comments standard out too much. (Hat Tip: @Alex)
They make the comments seem more disjointed and unrelated than they need to.
Suggestion from a design ignoramus: Give them wider margins, so they don't extend past the margins of the comments and/or make them a lighter grey.