Most people have little interest in a sporting event where one team is vastly better than the other - the $200 million Yankees versus the $40 million Royals, for example, or a lion versus an unarmed slave in the Roman Colosseum back in the day.
But games often have two distinct subgames - e.g., offense versus defense, or (in tennis) server versus returner. If the two subgames are wildly one-sided but each contestant has the huge advantage in one of them, the overall contest will be close. But it's still potentially boring, in that you know how each matchup within the game is going to end up almost all the time.
To my mind, this flaw is shared by Wimbledon men's tennis (where Isner just beat Mahut in the fifth set, 70 to 68) and the World Cup, where the offense generally seems to have almost no chance of scoring. On grass courts, returners often are hopelessly outmatched in the men's game. And amusingly crazy though the result was in Isner-Mahut, I'd say it's far better to have a close match where there's, say, a 25% chance of a service break each time around. Last year's Federer-Roddick final, though entertaining, helps make the point - the servers were simply too dominant. (And this is an old problem - Edberg once lost the Wimbledon final in 4 sets with only one break of serve - by him - in the entire match.)
In the World Cup, the fact that you can go away for an hour (if you're watching it to begin with) and be almost certain nothing will happen is, to my mind, a serious defect that prevents me from getting at all interested. You see a team trying to move towards the enemy goal, but know that, if you watch it on offense for a minute, its chance of scoring is probably less than 1 percent. So why bother to watch? If the rules were changed somehow so that an average score was, say, 4 to 3 (like hockey), I think it would be a vastly more entertaining game.
Then again, perhaps such jejune impatience is only to be expected of an American.
UPDATE: Uh-oh. From the Wimbledon website: "Isner's reward is a second round meeting with Dutchman Thiemo De Bakker, himself the winner of a marathon first match when he beat Colombia's Santiago Giraldo 16-14 in the deciding set. Perhaps they should be made to play best of three sets."
FURTHER UPDATE: De Bakker beats Isner in straight sets, 0, 3, and 2. I guess Isner left it all on the court on Wednesday.