I started playing call of duty on Black Ops last year. The only problem on Black ops was that nobody could join me. How come? I don't know. Most of the time I had an open NAT. Only if I had moderate NAT my friends could join me.
After completing Black Ops multiplayer i bought MW2 because my friends recommended me the previous game in the series. On this game I couldn't even play unless my NAT type was Moderate. Having a moderate connection gets me to play MW2, open doesn't work! From then on i went back to playing Black Ops. The times I had a Moderate connection I switched to MW2.
Now today I bought MW3, Guess what? The same problem still exists. As all my friends are playing online and i'm not I guess Treyarch isn't the problem.
IF NAT OPEN = Connecting to server
IF NAT MODERATE = Able to play
Putting my playstation into a DMZ didn't work either. I have also tried putting my firewall down to get a constant moderate nat type but this didn't work either. I have a d-link router (worth 135€) and i'm from Europe, Belgium. My connection is wireless and I always have 100% connection.