I'm so glad you shared your experiences with this. I've been warning about the accessibility issues caused by the Discussions Redesign for months now. I'm surprised they haven't resolved this very important issue yet. Given the requirements of the Americans with Disabilities Act, I'm actually kind of shocked that they are not required to make their features ADA-compliant and comply with Web Content Accessibility Guidelines (WCAG). Font that is too small to read seems like a compliance issue.
For those who want to read more about the ADA: The Americans with Disabilities Act | ADA.gov
For those interested in WCAG: WCAG 101: Understanding the Web Content Accessibility Guidelines | WCAG
Issues with text size and formatting can create a huge barrier for students and educators with disabilities to participate fully in asynchronous conversations in discussion boards. Based on my experience with the updated interface, formatting issues are also challenging for people who do not have disabilities because it doesn't adequately differentiate posts from replies in the threads.
Cheryl