Generally speaking games at cons DO suck. Well, unless you are in mine.
See, the trick, I have discovered, is this: pick the right game to run (or play in) and make sure you, as the GM, know what you are doing. Frankly, I do not think many GMs put the proper thought into what they are running. Let me give some examples.
First of all, D&D is a real trap. The standard game is about 4 hours. What can you do in D&D for four hours? I suppose you could do lots of hack & slash which would be fun if that is all you want to do. But you really can't do much of an epic adventure. Maybe you could solve a mystery or something but then many people do not come to do that. This is why I would not run D&D at a con and I have not PLAYED D&D at a con in ten years.
Now, there are some games which lend themselves to being played at cons. Great games to get into would be Call of Cthulhu (All Flesh works here, too), Paranoia, any supers game or any spy game. (Of course, if you hate these games then that is its own problem.) But these games can have fairly simply plots or can really hinge on things other than hack and slash. I think that is the reason why Call of Cthulhu and Paranoia are so popular at cons - seasoned players know they can have a great time with them.
Another trend I have noticed for popular games is the 'gimmick' game. These tend to be very popular. Some examples of the 'gimmick' game . . .
I played in a game called 'Cereal Killers' a few years ago which was filled and had to turn away generics. The guy had been doing this for years (and continues to do it). He makes Champions stats for Kelloggs and General Mills cereal icons and creates an adventure. Honestly, it is not only funny but he has a great plot that goes with it. It was a great experience.
This year the table next to me one day was playing Toy Story vs. Muppets. These people were having a blast and really getting into it. I am not sure what system they were using (maybe Champions/Hero) but could not be sure. The amont of energy, excitement and role playing at that table told me it was a total success.
So in short, I think you have to pick the right game and read the description carefully (and pray that the GM has written the description carefully).
The past couple of years I have been taking my own advice to heart and really working on games that can be done in just four hours and have a great amount of excitement to them. In fact, Saturday night I always host a playtest with some dear friends to see if the game I run would be good to fit into my annual rotation of games to run at Origins and GenCon. Some really great games have come out of that playtest session.