Since Scott Forstall's departure, Jony Ive, Apple's SVP of Industrial Design, has been put in charge of Apple's Human Interface team where he will provide direction for the look and feel of all of Apple's software.
According to rumors, Jony has been said to favor a more "flat" design and wants to simplify iOS. Many sources within Apple have said that iOS 7 is iOS "de-glitzed". There are also rumors that apart from aesthetic changes, there will be a new Mail and Calendar app.
Considering all this, it seems like iOS 7 would be a major and substantial overhaul. As a result, the iOS team is pulling engineers from the OS X team to ensure it meets its deadline, which is widely expected to be September, when Apple will also announce a new iPhone.
Time constraints aside, Apple is also under tremendous pressure to come up with something dramatically different and new. The company's recent releases such as the new iMac and even the iPad Mini has been ho-hum by comparison to the earlier iPhones and iPads.