I would have to agree its harder to read. As far as why, I think it has to do with the color of the text, as well as a change in the font and size. The old font had a heavier line quality (almost like a "bold" version of the current type), so it stood out from the background colors better.
If the thread headings were one size bigger, and the color of the font changed to black for the headings, I think it would help quite a bit.