What ports need to be opened to use the L2TP VPN server on Mountain Lion Server?
According to Apple, L2TP is currently inoperative when the VPN server is NATed. Looks like we have to wait for a bug fix, or use PPTP in the mean time despite it's much lower security.

Remember, if you configure PPTP, you need to activate OpenDirectory, and configure users there. Local users CANNOT use PPTP.

References:
http://support.apple.com/kb/TS5313
http://support.apple.com/kb/HT4748