I have a couple of accessibility concerns.
First: Users, for myriad reasons, such as ancient hardware or non first-world connections, often disable JavaScript. (Looking at the discussion at en:Wikipedia_talk:Notifications/New_message_indicator#The_bigger_issue_here:_Javascript, I can count at least six frequent editors whom this will affect.) I can say that, while this page is not impossible to use with JavaScript disabled, it is nearly unreadable. Every section displays as one undifferentiated blob of text. Nevermind, this is LiquidThreads, not Flow.
Furthermore, I worry about blind users with screen readers; from what I've seen, they have had challenges using some of the recently rolled-out changes, and it would be depressing if Flow didn't work properly for them.
Lastly, I worry that this will affect users with slow connections. Wikipedia is used throughout the world, but it is my understanding that broadband connections are only generally available in non-rural parts of North America, Europe, and Asia.