It's worth mentioning that Lee Sedol mentioned in an interview that even if he loses a single game against AlphaGo, he will have lost the match. He was expecting to win all 5 games.

As a ~1 dan amateur, whether the game was "really, really close" was not clear to me. For one of my games, yeah it was close, but professional play is on such another level I'm not sure how close this game should be considered.

I watched two 9d pro commentaries, Redmond's and Kim Myungwan's. Redmond was obviously being charitable in saying the game was close near the end. Myungwan said the victory was apparent several moves before the resignation, and Myungwan also said AlphaGo was clearly stronger than himself.

Either way, even if this game should be considered close, it's still not clear if AlphaGo was holding back in order to hold a secure win. It's possible it can play at a higher level, but it wasn't needed. We can't really know AlphaGo's strength until (if) it is beaten. The following matches will be very interesting.

Jumping the shark and jumping the gun are two different things. You want the second one.

Yes, but it's still a historic first. It's the first time a machine wins a game against a top human player.