Kakaroto, since this is a sticky, you might want to make the answers here as clear and thorough as possible, since people have still been posting on these webcam and port topics.
As kjir said more clearly in the thread "No more webcam support after I've changed my Netgear router":
"The answer, which has been given over and over, is that you need to have a port open on either side, so either you or your contact should have a TCP port open for direct connection. This is due to a bug in WLM."
This might be a useful distinguishing hint for some people (like me) who are used to their routers using SPI at the application layer to maintain sessions initiated locally on an outgoing port. I assume that this behavior is insufficient with a direct connection in aMSN, requiring an explicit opening of the port on one end of the direct connect?