In 2010, Microsoft officially announced during its Global Exchange event that it was going to have a new slogan for the company: Be what's next. The new tagline seemed to be well received at the time but now Microsoft has quietly abandoned the trademark for that slogan.
A quick check in the U.S. Patent and Trademark Office's website reveals that Microsoft allowed the "Microsoft Be what's next" trademark to expire on February 18th. Naturally, there's no word on why Microsoft decided to abandon its trademark on the site.
Even though the company introduced the new slogan in 2010, Microsoft ended up using the "Be what's next" tagline very rarely in its marketing and promotional materials afterwards. In August 2012, ahead of the launch of Windows 8, Microsoft launched a new design of its entire logo for the first time in 25 years and the "Be what's next" subtitle was not included in the rebranding.
The USPTO website has no indication that Microsoft plans to offer up a new slogan anytime soon. It sounds like the company feels, at least for the moment, that Microsoft is doing just fine without a catchy marketing subtitle.
Source: USPTO website | Image via Microsoft