To start, I like the new design in terms of color-themes, fonts (I've always been a fan of Roboto) and usability; it looks fantastic! That said, the lightness of font-family/low font-weight is making it harder to read questions, to navigate through the list of questions, and especially to read comments because they are a slightly smaller font-size. I am not sure that I have a great suggestion but here are a couple of ideas:
Giving certain things (Comments, Questions-List, Question-Page text) a heavier font-weight/larger font-size or possibly a different color that would stand out more.
Utilizing background colors to make the existing text stand out a little more
Reverting to the previous font or using
Arial, "Helvetica Neue", Helvetica, sans-serif;like Stack Overflow does.
I don't know if this is the correct forum to start a discussion about this (or if there even is one), and I am certainly not saying anything negative about the new design in general, but I am sure that we can all agree which of the following two screen-snips is easier to read: