John Siracusa’s 24-page review of OS X Mountain Lion:

Mountain Lion is not the Mac OS of the past, but it also sets a course to a destination that is quite distinct from iOS. Despite the oft-cited prediction that Mac will eventually be subsumed by iOS, that’s not what’s happening here. Apple is determined to bring the benefits of iOS to the Mac, but it’s equally determined to do so in a way that preserves the strengths of the Mac platform.

Gruber makes a similar point in his review:

Mountain Lion and iOS 6 certainly share a slew of features and code […] But they are very much two different and distinct systems, one for traditional keyboard and pointer device personal computers, and another for touchscreen mobile devices.