I think it's going to be a combo of the new design and the bugginess of the GM. While I imagine a good number of people will like the new look, a bunch more will not and Apple has done very little to prepare them for the radical changes that will be coming when they click that update button. The average iPhone user does not pay attention to Apple keynotes. Both my parents and my grandmother had no idea whatsoever that iOS 7 was different or coming next Thursday.
If that was the only problem, Apple could probably work through it. Mind you, they could have saved themselves a lot of trouble through a more concerted campaign to educate their customers about the changes that were coming. But, they would make it. However, the GM is shockingly buggy for a public release. I have run into more bugs in the past day with the GM on my iPhone 5 than I did in 3 weeks with beta 6. And the situation is even worse on the iPad. I'm really surprised Apple is allowing the software to be released in this state (especially the iPad version).
If it was just the new look, or just the bugs, I think Apple would be ok. I don't know how consumers are going to feel about a radical redesign that is also buggy. I know I'm not super thrilled.