The other nice thing about the missions adventures is that you can play a character with some experience at a con (or if you are lucky enough to have a firebase near you there also) and know that the player next to you had to work for the experience, gear and perks his character got not just handed to him by some Monty Hall GM. Also it is a somewhat controlled world to keep a more even keel for the game base.
That being said there is now a huge amount of Karma disparity between some players who have characters dating back several years to the Denver (or even first Seattle) series of missions and the characters that only have one or two missions under their belts. Not only does this give them a bit of an advantage and a tendency to sometimes (though not intentionally in my experience) take over a game, but makes it harder for a GM to run a fair, balanced game that challenges all the players and in which everyone has fun (the two main goals of every game I run).
I am not saying that these players shouldn't be allowed to continue to play these characters. If anything I would like to see grander adventures to challenge them more. I know the table rating system allows for this compensation to an extent, and is the purpose behind it, but I think it would be cool to at each convention have a major mission (maybe run 3 or more times through the convention) that is solely for the 50+ or 100+ karma characters.
By the same token it might be nice to have a game or two of pure starting characters (or under 20 karma) so they have a chance to shine and everyone is truely on even footing.