Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Actually, you still can. You just need to swipe from the edge of the screen. That's been standardized in iOS 7, and all apps are slowly adapting that design.

Functionality did change as I can no longer swipe left to right to move back a screen.
 
The Instagram redesign takes lots of cues from the Facebook redesign, which is expected. Maybe they wanted to unify the look of their apps?

I like Instagram's new look, even though it looks like they just flattened everything. It looks great on my iPad Mini, which can finally use the Retina assets inside iPhone-only apps.

I feel that apps should still have the creative license to deviate somewhat from the look of the OS. I wouldn't want every app to look like iOS 7's Mail or Safari.
 
I love the new instagram with the iOS7. The clarity and picture is amazing on it now.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.