"It's an extra burden on short-term memory," he said, referring to the need to open one app, remember what information was there, or even where that information was, then apply it to another app. "Short term memory is notoriously unreliable. Even something you'd do at home, maybe research a vacation, is difficult to do because it requires comparing and collecting information across multiple windows."
Microsoft's biggest mistake may have been to shoehorn two UIs into a single operating system, but it made a raft of other choices that compounded difficulties for customers.
The charms -- generic command icons that appear at the right when the mouse is clicked at the lower corner, or the user swipes from that side -- are, Nielsen said, "a good idea," but they're easily forgettable by typical users. "The problem is, 'Out of sight, out of mind,'" he said, noting that the testers often forgot to summon them, and then struggled with what do next.
The idea of hidden commands permeates Windows 8 and Windows RT, and although it's smart UI design for a space-constrained screen, like a smartphone, it makes much less sense on a tablet and no sense at all on a PC, he argued. Yet Microsoft rolled it out across all platforms.
Worse, it runs counter to other Microsoft projects -- Nielsen pointed to the "ribbon" that debuted in the company's Office 2007 -- and to the way most websites work.
"The Web relies on 'just-in-time visibility,'" said Nielsen, speaking of well-designed sites that show commands and features in the context, and only in the context, of when they're needed. Users have become "trained," for lack of a better word, to expect that.
"You simply can't design a website with hidden features and expect it to be used," he said.
Some, however, have disagreed with the critics, if not Nielson specifically, who have called Windows 8's UIs into question. They've maintained that users will get used to the usability foibles of Windows 8, or if they can't, that they should just suck it up and learn to live with them.
Those defenses don't hold water with Nielsen. "A year from now, certainly, Windows 8 will seem easier than it was the first week," he admitted. "But at the same time, there will always be these problems. People must think to do something, rather than being reminded to do something, and they will always neglect useful features."
That's no way to be productive, which in the end, is what most people expect from their traditional computers, and increasingly from their tablets.
He also rejected the notion that because users adapted to the shift 17 year ago from DOS to Windows 95 that they will gladly do the same this time with Windows 8.
"The difference is that then they took something really bad, DOS, and added something, Windows, on top of it that was much easier to use," Nielsen said. "This time they're taking the standard GUI [graphical user interface] that has a lot of usability and discoverability, and making a U-turn by hiding features."
The result is a user adrift from the hard-won experience gained through years of time spent working with Windows.
"With Windows 8, you don't feel in control," said Nielsen. "One of the biggest goals of user interface design is to give people the feeling of mastery or control. This is a big, big change. Users have become familiar with the idea that the 'mouse is me,' but Windows 8 largely discards that. People feel a loss of control, and feel insecure in relation to the machine.
"That's the failure, and the missed strategic decision," Nielsen said.
Microsoft has, of course, trumpeted the massive UI changes as a step in the correct direction, not a wrong turn, calling Windows 8 "fast and fluid," labeling it a "no-compromise" solution, and dismissing criticism that Windows 8 is difficult to use.