I'm guessing you only did Single Port Forwarding, and if I'm correct you need to do Port Range Forwarding.
The Start port should be 28000 and end port should be to 28030
This has not been true for a while; only 1 port is required.
UPnP, needs to be enabled. (SIMPLE STUFF)
Not true, you can port forward instead.
As has been suggested earlier, talk over IRC and see if they are able to connect; don't assume just because no-body joins your server they can't.