Part of the reason for the muted response, I think, is that the audience consisted of members of the press, and they will show restraint. This 'media event' is really an advertisement for new products. Applause after the various software demos was rather muted as well. This is a different audience from that which attends WWDC and Macworld Expo. Those audiences are composed of enthusiasts, developers, technologists, etc. covering a broader demographic, and they as a whole show more emotion.
Also, when you pack a few thousand people into a large auditorium you get a different level of energy than with a couple of hundred in a small intimate setting.
Personally I think the new iMac design is very pleasing. I went to an Apple store to see it in person and it looks outstanding. However, it's not a revolutionary change and does not cause jaws to drop.