I am a blind user and this is what I have to say:
Keep it Simple! Stupid upgrades and updates people do NOT want will not incite an influx of new users. Be loyal to the ones who are happy with Yahoo email and like the simplicity in layout and use of Classic/Basic email. even the "new" Basic email sucks in comparison and I can't get it back to classic and it is annoying the **** out of me. Because of the new updates, I hade started migrating some of my email usage to Gmail which has started including features that were native to classic emails. So the "newness" of the features in gmail that had prevented me from moving to gmail usage has now become the bane of yahoo and gmail is re-adopting "basic" features/buttons/appearance. Don't be stupid and go through a cycle of learning that will just drive useres away from yahoo. Keep it simple.