This is looking pretty great. Super clean. I'm slightly less enthused about the gray background on the first comment (correction, on the thread title bar/box). It looks a little, muted. Grayed out background is often for archived, old, past content, and to me it suggests lowered importance and attention is due. Maybe a different tone or hue would do the trick. Great work, keep it coming!
Now I see that I'm supposed to enter a title. The nice superbold title text helps a lot, so I largely retract the above comment. I'm still not crazy about the gray text on grayish background, it feels slightly light and low-readability for me.
Also a note on the "Comment" link. It's not clear to me that the Comment (#) is intended as an active link to "leave a comment". It feels like clicking it will link to previous comments but not to adding a new one. Perhaps that's because the (#) makes it feel like "Comments" as in link-to-comments rather than "Comment" as in "Leave a comment". The combination of "Action"(number) pulls my attention in two different ways.
I see that Comment (#) becomes Comments (#) if comments > 1. This still leaves me confused, though it's a nice touch in terms of responsive UI. I prefer active links (do this) to be static text.