Given the 1 vs. 128, 2 vs. 127...64 vs. 65 pairings formula FIDE uses for the World Cup, significant upsets are relatively unlikely. In fact, only one of the 38 players rated over 2700 lost, and that was 16th seed Alexander Morozevich, who lost with Black to 2524-rated Canadian GM Bator Sambuev. Morozevich was in good shape in a complicated position, but a serious error late in the game turned the tables.
A bit below the 2700 mark there were some notable upsets though. Judit Polgar (2696) lost to Isan Reynaldo Ortiz Suarez, Vladimir Akopian (2691) lost to Ngoc Truong Son Nguyen, and Andrei Volokitin (2688) lost to a player with only two names - Ray Robson of the U.S.A.
Transitioning to the other American results, Nakamura won pretty easily against his opponent, rated more than 300 points below him, but two boards down Gata Kamsky only managed a draw (albeit with Black) against Lou Yiping. Alexander Onischuk defeated Eduardo Iturrizaga, and in a semi-upset Alejandro Ramirez drew with the 2700+ rated Evgeny Tomashevsky, though with White.
That was the good news for the U.S.; now for the bad news. The four other Americans all lost to 2700+ rated opposition: Larry Christiansen, Gregory Kaidanov, Alexander Shabalov and Conrad Holt lost to Laurent Fressinet, Alexander Areshchenko, Maxime Vachier-Lagrave and Nikita Vitiugov, respectively.
With 64 games to choose from there were plenty deserving careful attention, but the interests of brevity I'll note just three, which you can replay here. Two have been mentioned already: Morozevich's upset loss and Robson's impressive win, in which he built on good preparation with a very nice combination. The third game I'll note is the young Russian star Daniil Dubov's win over Sergey Fedorchuk, which ended with a spectacular mating combination.
Just a reminder: day 2 continues the first round, as the players switch colors and do it all over again. All players who are ahead after day 2 get the next day off; those who are tied play rapid (and if necessary, blitz) tiebreaks to decide who advances to the next round. Full results here, or more conveniently, here.