Maybe I misinterpreted what you are saying, but if you change their default gateway and block all ports except the one for your game, then you effectively shut off their internet while playing. For me, and I think most people, this would be a huge negative.
Your assessment of the tech is correct, but I don't think your assessment of the user-behavior is..
Keep in mind, this is for a full-screen game, and you can't really play the game and do other things. (This is one of many reasons why we're re-writing it to be web-based)
I think that when it comes to behavior on this subject, there are really two types of users..
The users who are bittorrent downloading, streaming media and other things where cutting out the internet connection would matter.. And the others, who just want to do some homework, then play a game for a while.
For the people who just want to play for a while, having the rest of their internet cut out won't be a major issue.. If they're playing the game, they can't browse the web, or use AIM, but since it's full-screen, they wouldn't be able to do those things anyway.
For the other folk, who use their connection heavily downloading torrents (legally), and listening to streaming radio- They're far more likely to be technically savvy enough to forward the ports necessary to make the game work, and wouldn't require any sort of VPN at all.
Truth be told, we probably could fix the background download problem.. If we recorded every IP that tried to contact your public IP, and then set your gateway to be through the VPN when trying to reach THAT IP, it just might work..
The thing is, that would be fragile, and might take a week or two of work to get right.. Since this is a legacy application, and we're actively working to replace it, for now I'm happy with the 90% solution. I'd rather spend the two weeks making version 2.0 closer to launch.
