I guess the main thing current non-Firefox users are looking for is... for UI elements to be inexplicably rounded (and for the rounded contents within them to not have appropriately-matched radii, so it all looks out of place) and for tabs to look really small (and rounded, of course) but actually the "whitespace" they've added so you can see they're rounded is still clickable? The new default theme and it's variants don't appear to have any way to revert to a solid colour, it's now an extremely distracting gradient all the way across the main toolbar.
edit: set browser.nova.enable to false in about:config