Ubuntu changed the completion scripts in order to be "more smart". That is, it completes entries from /etc/fstab only. Now, not everyone puts mount points in /etc/fstab and it's just annoying that I cannot complete from /dev/disk/by-label anymore nor loop mounts of .iso files.
To revert to the old behavior, edit /etc/bash_completion.d/mount and change:
complete -F _mount -o default -o dirnames mount
By doing so, you loose the completions of /etc/fstab, but at least it allows you to complete files and directories.
complete -F _longopt -o filenames mount