In his opinion, the operating system interface is "a monster that terrorizes poor office workers and strangles their productivity." However, on a positive note, he stated that "there's nothing that a modest redesign can't fix".
Nielsen's analysis is based on a usability report he conducted with both inexperienced and experienced users that were confronted with Windows 8 for the first time and had to complete specific tasks. Much of his criticism is based on the notion that Microsoft went to an extreme approach to adapt a touch UI that we love to use on smartphones, but may not be such a great idea on a larger PC screen. "The new look sacrifices usability on the altar of looking different than traditional GUIs," Nielsen wrote.
From the post:
"Windows" no longer supports multiple windows on the screen. Win8 does have an option to temporarily show a second area in a small part of the screen, but none of our test users were able to make this work. Also, the main UI restricts users to a single window, so the product ought to be renamed "Microsoft Window. When users can't view several windows simultaneously, they must keep information from one window in short-term memory while they activate another window. This is problematic for two reasons. First, human short-term memory is notoriously weak, and second, the very task of having to manipulate a window—instead of simply glancing at one that's already open—further taxes the user's cognitive resources."
The designer also took issue with the fact that Microsoft chose a path to a flat surface and clickable icons are more difficult to identify, the fact that charms are integrated in an "out of sight, out of mind", which causes people to forget that they exist:
"In practice, the charms work poorly — at least for new users. The old saying, out of sight, out of mind, turned out to be accurate. Because the charms are hidden, our users often forgot to summon them, even when they needed them. Hiding commands and other GUI chrome makes sense on small mobile phones. It makes less sense on bigger tablet screens. And it makes no sense at all on huge PC screens."
According to Nielsen, touch gestures could also use another thought or two:
"The UI is littered with swipe ambiguity, where similar (or identical) gestures have different outcomes depending on subtle details in how they're activated or executed. For example, start swiping from the right to the left and you will either scroll the screen horizontally or reveal the charm bar, depending on exactly where your finger first touched the screen. This was very confusing to the users in our study."
To improve Windows, Nielsen suggested to revise application guidelines to promote a more restrained use of Live Tiles, consider higher information density, better visibility of key features, "and many other usability guidelines we've already discovered in testing other tablets."