I think Frodus makes a good point with DDR, a game that combines physical and mental dexterity. DDR can't really be lumped in with other games in this discussion, so when I refer to "competitive gaming" I am referring to the Halos, Smash Bros, counterstrikes, and so on.
I also remember someone once saying video games can never be as enjoyable to watch as a sport because it is difficult to portray the human drama of winning/losing in a video game. As was previously mentioned, it's not much fun to sit there and watch someone move their fingers around. At the same time, it's not really all that entertaining watching a video game character run through a virtual world, because you miss the human aspect of it. I don't know if this problem can be reconciled through a new approach in broadcasting competitive video game matches, but I think it is something that has to be considered if competitive leagues are ever going to really pierce a North American mainstream audience.
I also think that the number of games there are hurt the chances of someone willing to watch other people play. Frodus already pointed out Hockey as a niche market, because in the states it certainly is. A big part of the enjoyment of watching a sport is realizing how skilled and talented athletes are because you have tried to play the sport they are so adept at. Not nearly as many Americans grow up playing hockey as Canadians do, and so Canadians find it far more entertaining than the average American, because they have a point of reference with which they can understand and appreciate the level of competition they're watching. With video games, no one plays any one game for that long, a newer version of it is always only a couple years away. And even then, not everyone plays the same games. I can't appreciate a good battle in Starcraft, but I can tell you when someone in Command and Conquer gets owned. But even if you can't appreciate hockey because you've never played it, everyone has done some physically exerting activity or played team sports growing up, and the athletic competition is a universal reference point.
Until games can overcome the virtual character/human player gap, and build upon a universal appreciation of skilled gaming (as has already happened in Korea with Starcraft) I don't think games can be put on the same level as a sport.