What the summary says. David, do you want to find someone to fix this?
It just landed on Central. I'll give it a try and report back here with findings.
The one problem I'm seeing with the new tab page feature is that the buttons to pin and remove a site are not keyboard accessible. In browser/base/content/newTab/grid.js, they're being created with a tabindex of -1, which makes them non-tabbable. The links and buttons themselves are exposed to assistive technologies, so is the show/hide button, so all is good on that end except for this one piece.
Sorry, forgot to mention that the pin/remove buttons are intentionally unfocusable for now. They will be once bug 716532 is fixed (which we'll do before pref'ing it on by default for Nightly).
Will be fixed by bug 729878.
Fixed by bug 729878.