One reason that can explain Sagan's domination right now is because the old guard is out of it for some reason.
Oscar Freire, who usually is very good at these kind of stages, is getting old and will retire soon.
Fabian Cancellara crashed during the Ronde and is still far away from his best shape. Remember the Tour de Suisse 2008? Now that was truly impressive.
Philippe Gilbert has been bad all year. I don't know why, but he's out as well this year for some reason...
Tom Boonen seems to have his mind on 3 races this season: the Belgian nats, the Olympics and the WC. Back when he was young he always raced to win.
Edvald Boasson Hagen could have definitely challenged Peter Sagan here, but he rode the Dauphiné instead. He seems to be stuck on domestique duty most of the time anyway, what a waste of talent.
Thor Hushovd has been crap all year as well, but last year he could still challenge Sagan on stages like this. Like Oscar Freire, he's also getting old.
Valverde isn't the same anymore after his doping ban. Will he ever return to his old level? Only time will say.
He's also the first super star of a new generation. Just like Boonen was in 2005. We all thought he'd dominate for years, but then came Fabian Cancellara, Filippo Pozzato, etc Who knows, Sagan's biggest rival may be lurking behind the corner as we speak.