Microsoft’s Windows 10 Displays a Familiar Look - NYTimes.com

Perhaps the "10" denotes the number of years during which Microsoft has largely lost client OS competitive ground since the termination of Windows Longhorn

"With Windows 10, Microsoft wants to give business customers the opportunity to provide input on the software before it is finished, said Terry Myerson, executive vice president of Microsoft’s operating system group. The product will not be released in final form until the latter half of next year.

“It’s a little bit of a journey,” Mr. Myerson said. “We decided to jump off today before we had all the answers.”"

Microsoft’s Windows 10 Displays a Familiar Look - NYTimes.com