Per the Daily Tech article:

  

New cross-platform OS project is unprecedented, arguably largest software project in history

A new report in The Seattle Times interviews some of Microsoft Corp.'s (MSFT) top executives who describe the company's inspired new design direction.  
 
With Windows 9, Microsoft is plotting quite literally the largest software project in history; combining operating systems from at least five platforms, cloud services, and dozens of software projects into one tightly integrated cross-platform bundle of software.
 
It's an incredible vision.  And it's one that could change the entire industry.
 
I. The Path to Unification
 
A major part of why Windows 8 had so many rough edges was because it was an exercise in porting.  But much as Windows Vista was -- in some ways -- a necessary stepping-stone to the more polished and beloved Windows 7, Windows 8 (and 8.1) was necessary as a stepping-stone on the path towards unification.
 
There was always a fair amount of code exchange between the mobile and PC OS trees of Windows.  But in the era of Windows Mobile (2000-2010) that process was more infrequent and sporadic, with development of the two branches largely independent.  With the launch of Windows Phone in 2010, Microsoft found its design direction -- the Modern (Metro) UI.


 
With Windows 8 and Windows RT, Microsoft looked to have greater code sharing between its various products.  But internally the situation had not substantially improved.  While Microsoft had a unified design and a number of share core features across its platforms, it now had four separate code trees -- Xbox, Windows RT, Windows, and Windows Phone.  Each platform's OS group had a separate design and software team.  When a change was made to core APIs or Modern UI in one of these platforms, it was a time consuming process to port them to the others.  
 
The first change came late last year with a leadership shakeup that unified Microsoft's OS design teams under a single common banner.  With Windows 8.1 and Windows Phone 8.1 (and the Xbox One), Microsoft began the Herculean task of unifying pieces of its API.  But much work remained.

That work is reportedly culminating with Windows 9, Microsoft's first family of consumer-facing operating systems (OSes) to feature fully unified and synchronous development.  According to The Seattle Times, Windows 9 will be a watershed release for Microsoft.
 
On the backend, Microsoft is approaching maximum unification for APIs.  Form factors (touch, small screens, big screen TVs, etc.) mandate some specialist code, but for the most part Microsoft is reportedly aiming to give customers one look and feel across the smartphone, Xbox, tablet, PC, and tabletop computer (Microsoft's Perceptive Pixel offerings).
 
II. The Men Behind Microsoft's Moonshot
 
Two corporate vice presidents are leading the unification effort.  On the software front, David Treadwell, 47, is leading the update.  On the user interface (UI) front, Joey Belfiore (a veteran manager of the Windows Phone team), 46, is leading the effort.
 
Both VPs report to Terry Myerson, executive vice president of Microsoft's Operating Systems Group.  In terms of consumer-facing products, one major result of the shifts in leadership was the emergence of the OS group as the leader in development direction -- a seemingly intuitive shift for a company who became a superpower thanks to its MS-DOS and Windows operating systems.

Mr. Myerson gushes about his two lieutenants:

From now on, says Mr. Treadwell, expect a release to bring updates to all of Microsoft's major platforms.  He describes this revolutionary approach -- which no other OS maker has achieved yet -- stating:

 
And these efforts weren’t limited just to the OS developers.  They also worked with Microsoft's software and enterprise teams, including the teams responsible for Azure, Office, Bing, and Skype.  The result is that Windows 9 should be giving each Microsoft software project the tools it needs to create a next generation experience.

The software side of things has already come to bear with Microsoft's "Universal Apps", which offer quick porting of a single app across the PC, (HD) tablet, smartphone, and Xbox, with common permissions and bundled customer purchase options.
 
III. Why Windows 9 is a Game Changer
 
During Steve Ballmer's 14-year reign as CEO of Microsoft, there were some high notes, but in terms of market direction Microsoft undoubtedly lost some of its glow to consumers and investors.  Companies like Apple, Inc. (AAPL) and Google Inc. (GOOG) emerged as real threats to Microsoft's hegemony of personal computing.
 
But with Satya Nadella's tireless commitment to cloud-back services and a unified Microsoft -- his "One Microsoft" vision -- Microsoft appears poised to be producing the most massive and efficient multi-platform project in history.

Both Apple and Google are headed in a similar direction. OS X 10.10 is expected to take design cues from iOS 7.  But neither company appears as far along as Microsoft.  What Microsoft is planning -- a singular cross platform update for large computers, consoles, PCs, tablets, and smartphones -- is unprecedented.  It's never been done.  Microsoft is also working to tightly integrate its packed stable of consumer software offerings into these updates, and it's an incredible vision from a technical perspective.

While casual consumers can look forward to a higher degree of polish, there's plenty for power users and enthusiasts to eagerly await, as well.  Microsoft has already stated that either Windows 8.2 (an interim release) or Windows 9 will feature the return of the Start Menu to Desktop Mode.  What's more, Microsoft engineers have hinted that Windows 9 may at last introduce multiple switchable desktops, a much beloved feature from Linux.

_{Microsoft is bringing the sexy Start Menu back.  [Image Source: Redmond Pie]}

 
Windows 9 is on pace for an April 2015 release.  Windows for tablets and smartphones is expected to continue to be free to OEMs, a major edge over Google's Android which brings licensing fees of $5-15 USD per device to Microsoft.

Needless to say, if Microsoft delivers what its promising with Windows 9, it could be a game changer for the entire industry, the effects of which could be felt for decades to come.

Source : http://www.dailytech.com/Microsoft+Moonshot+Windows+9+Will+Unify+PC+Xbox+Phone+and+Tablet+OSes/article34992.htm

I hope its not the garbage that W8 was o-o

Meh. I am still happy enough with good ol' Windows 7!

As an owner of multiple PC's/Laptops that run Windows 7 and 8. Ialso have a few Surface tablets and a few Windows Phones. I also own all of the xbox consoles(and Nintendo/PlayStation consoles less the PS4). I have had Mac PC's and laptops and own ipads too. I can't wait for one OS to link them all!!! That would be worth it just for the sheer ease of use...

Have you experienced Windows 8.1 update 1 it is one of the best windows. Very fast boot up, for me my start screen is awesome, the music and video apps are good. Even on my gaming pc besides big titles I am getting store apps and games. I love it and if you have a touchpad it is an experience like none other.

ONE OS TO RULE THEM ALL

MS won't make two bad OS in a row. 

Great to hear being an owner of Windows 8!

so they are going to keep going on this unification of all devices approach..

..i don't have any particular love for apple but i guess that's going to be the only via platform for a while.

This is a terrible strategy on Microsoft's part. The devices need OSes that are designed for their form factor. If they try to clamp everything together under one OS, it will most certainly give off different experiences for each. Those experiences will dilute the total experience for each device, therefore making each device crappy. Windows 8 and Windows Phone 7/8 has done this very thing, and the sales show the level of people's dissatisfaction for this design choice.