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Two years ago, when Microsoft debuted the Kinect sensor for Xbox 360, the company celebrated the launch on its website by replacing its standard corporate mug shots with Xbox Live avatars for all of its top executives — except for Windows president Steven Sinofsky, who was represented by a Windows logo instead.

It struck me as weird enough that I sent Sinofsky a lighthearted message about it: Where’s your avatar? He brushed me off with a friendly no comment.

Whatever the reason, for me it symbolized Sinofsky’s reputation inside Microsoft — focused intently on controlling the success of his own division, and not all that interested in playing along with the rest of the company.

That character trait seems to have been a big factor in Sinofsky’s departure from the company on Monday. Microsoft CEO Steve Ballmer’s quotes in the official news release are telling — talking about the “great integration of services such as Bing, Skype and Xbox across all our products” and saying “it is imperative that we continue to drive alignment across all Microsoft teams, and have more integrated and rapid development cycles for our offerings.”

In other words, Microsoft’s future will depend on the ability of its divisions to work together effectively. And Sinofsky is out.

Microsoft describes the decision as mutual, effective immediately.

I have to say I’m sorry to see him go. Say what you will about him, Sinofsky is the type of character who makes a company more interesting, at least from the outside. He deserves credit for turning around Microsoft’s flagship product, grabbing the wheel and getting the division back on track after the Windows Vista debacle. And with the development of Windows 8, he led a complete overhaul of the operating system for tablets and touch screens.

Sinofsky has dry sense of humor — even charming at times — wrapped in a prickly demeanor.

“Go learn something,” he said to a New York Times reporter, gently pushing him away after he asked a tough question at the Microsoft Surface unveiling. In an interview with CNet News in 2009, he greeted questions he didn’t want to answer with a silent, stone-faced stare.

At the same time, as I learned over the years, he has a great habit of dashing off private emails to reporters — short-circuiting the official Microsoft channels — when he has something he wants to say about something they’ve written, or something they should be writing about, at times unrelated to Microsoft.

The messages are frequently chiding in tone, but also constructive in spirit — genuinely trying to help the misguided recipient understand something. And more often than not, his points are dead on.

In his farewell email to his team, sent from his Surface, Sinofsky signaled that he won’t sit idle for long. “After more than 23 years working on a wide range of Microsoft products, I have decided to leave the company to seek new opportunities that build on these experiences. My passion for building products is as strong as ever and I look forward focusing my energy and creativity along similar lines.”

He also sought to head off speculation about his departure. “Some might notice a bit of chatter speculating about this decision or timing. I can assure you that none could be true as this was a personal and private choice that in no way reflects any speculation or theories one might read—about me, opportunity, the company or its leadership.”

Sinofsky also poked fun at his habit of writing extraordinarily long blog posts. “I have always promised myself when the right time came for me to change course, I would be brief, unlike one of my infamous short blog posts, and strive to be less memorable than the products and teams with which I have been proudly and humbly associated. The brevity of this announcement is simply a feature.”

I was never on board with the speculation that Sinofsky would be the next CEO of Microsoft, and with this news it’s pretty clear that my hunch was right, absent a triumphant return. But as the CEO of his own company, he would be formidable. It will be fascinating to see what he does next.

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