Nothing really, although you'll break anyone logging in as postgres or su'ing to that. And being a mount point, you just can't 'mv' it. What are you trying to do? See if remounting the drive will fix things? If that's the case, I'd copy the contents not move them, then delete everything under /var/lib/pgsql, but I wouldn't do an 'rm -fr' just a normal 'rm'.
