From Fedora Project Wiki

Is there a bug for NFSv4-related blocker items? NFSv4 introduces some hard to track down issues, primarily: 1) id mapping problems (which often give cryptic errors) and 2) unified filesystem (requiring all other trees to be bind-mounted under a filesystem with fsid=0, which is referred to by "/" instead of the full location). For NFSv4 to be the default, it seems that these issues should be addressed first. There are also other problems, like the fact that NFSv4 can't yet be used as a read-only root in Dracut. I think NFSv4 is a great improvement overall, but it's possible that the change will be very painful. --amcnabb