It looks quite pretty but there are some things bugging me about it, mostly from a "native" perspective (things that don't really fit in Windows IMO):
- By design, Aero has square inner window borders - Aero has a bevel highlight on all sides of the elements, not just the bottom - The search bars in Windows have always been square - Window title is styled differently to native Windows titles
I guess those can all be ignored becuse aren't trying to make the app fit in as a native app. It is more pseudo-Aero.
As for the real issues: - There's no scroll bar when there obviously should be (we still design for people with no scroll wheel ) - The second button from the left has a top scroll arrow that is bigger than the bottom one.
Apart from that, it is a very nice, neat, crisp design.
First four points you mentioned were intentionally designed that way, mostly to my tastes. I did struggle with point number 2 but that detail interfiered with the dark sidebar, I decided to break that "native rule" after noticing its inexistence in Google's Chrome browser.
I definetly didnt think about the scrollbar. The compose icon (I beleive you're reffering to), will be changed.