The implications are astounding, and I’m still wrapping my brain around the possibilities. For one, it could mean super fast in place load times, because Drupal doesn’t have to mess around with passing everything around the theming system. An AJAX request could just ask for the data, Drupal sends it, and jQuery will format your list, table, teaser, or what have you. In its second day of life, it already works with messages (and item lists, I believe).
A potential problem I see might be that slower computers would see slower rendering times. But this is already happening with some jQuery-heavy sites anyway. The benefit would be most seen with faster hardware, but this is always true with cutting-edge technology. Everyone else will catch up soon enough.