I'm finding it harder to read the reputation and badge counts on the new black top bar. The bold white text on the black background seems almost blurry. Also, I don't know if it is the 1px size difference or the new colors, but the badge counts are also slightly harder to read.
If you compare the design to the old top bar, the old reputation count is not bold, and it is much clearer. Also, the font size is 13px instead of 12px, and that one pixel does make a difference. The white colored text also stands out more than the silver or bronze colored text. (Sorry, I don't have a gold badge on Meta Stack Exchange yet, so my screenshot doesn't include any gold text :))
The new Stack Overflow top bar uses the same bold, colored and smaller text as the new black bar, but these changes don't seem as bad when on a white background. The bold black text and colored badge numbers are still clear on the white bar.