Microsoft sells more copies of Windows than anything else. It's the most popular piece of software in the world. That alone makes it "risky" to improve on because so many people depend on it. It's interesting that Steve calls it "the next release of Windows" rather than simply "Windows 8" but then again, Microsoft makes a point of not using a product's name, or even its codename, till after the official announcement. You can watch Ballmer's most recent interview, at the Gartner Symposium in Orlando, below:
Early details on Windows 8, which is expected to arrive in 2012, leaked in June 2010 but we wouldn't classify any of them as particularly risky. Highlights included USB 3.0 and Bluetooth 3.0 support, "instant on" booting, stereoscopic 3D, facial recognition as a security option, touch improvements for slates, and an app store.