It may be that Comcast blocks the RDP port for residential connections, though I thought they did not.
Can you double check your firewall and forwarding settings? Some routers require you to both forward the port AND add it as an allowed port.
*Edit* And as graysonf
said, if you're testing from an internal host, most commercial routers are not smart enough to allow connections to the external IP on the router from the LAN side.
But if you checked with an external web site, it sounds to me like the port forwarding and/or firewall rules are not working properly (or comcast is blocking that port, but I doubt it).