Contrast issues are definitely my biggest pet peeve. I can't believe so many designers aren't concerned that their content is unreadable. If you want to hide it, maybe it shouldn't be there in the first place. If you're not trying to hide it, then why is it so damn hard to read?

When scrolling down for a while, i find it hard to see if a comment is as the top level (no parent)

Well, I may have made a bad prediction there. We can't be sure though... :)