The NFS server on Wheezy GNU/kFreeBSD (freebsd-nfs-server) refuses connections from Wheezy GNU/Linux clients when the NFS version is not specified. It defaults to NFS v4 and the server refuses the mount request.
The solution is to add the -o vers=3 option to the mount command on the client, like this: $ sudo mount -t nfs -o vers=3 $SERVER_IP:/path/to/share /local/mount
Make sure to append -h 0.0.0.0 to the rpc.lockd arguments as well in its init file, as per 664812. fixed in wheezy!
Shutdown initscripts for NFS may be slow: see 664812
If you are using e.g. an OpenELEC client, you will need to append enable TCP connections to nfsd. Without it, mounting the NFS shares on OpenELEC will not work (tested with OpenELEC 2.0). You can do that by creating an /etc/default/nfsd file containing: DAEMON_ARGS="-t -u"
I also recommend the proto=tcp mount option on clients (which avoids path MTU issues, and more).