1) This is the first time they've ever done a show of this type--they had no idea how many people would show up, or even what types of acts they'd get.
2) No one knew how good the acts would be, so they probably passed through too many people and had to figure out a way to get a winner in the number of shows they're allowed.
3) We have a former Baywatch actor, a singer we haven't heard much from in a while, and an English guy judging. They are going to be biased!!! It's not entirely their fault, although they're adults and should be able to put at least some of their bias aside.
4) The show and the website only say that America decides the ultimate winner. Meaning that until the final episode, we don't have the only say in the matter. So until that last show, the judges and producers can do whatever they want to weed out the acts they don't like as much. The way they chose to do it was a private screening. Unless NBC decides to post the videos from that round of competition, we'll never know why certain acts got passed through and others got kicked off.
5) There are three more semifinal rounds! I think a lot of people have missed this detail. So if your favorite act wasn't on last night, be patient, as well as grateful that it didn't have to go up against the cloggers or ventriloquist yet.

Hope that clears up a lot of stuff, and hopefully next time they'll get the commercials right as to when it starts so we don't have as many people missing the first bit.