With the advent and developement of 3DTV as well as the developement of LCD TVs and Touch Screen Surfaces, Microsoft had to move away from 'Life Without Walls' as walls (or at least what hangs on them) are likely to be the next interface.
'Be What's Next' does conjure images of continued developement but that is not always a good thing. Back around 2000 'Be What's Next' with thoughts of Windows ME? No thanks... I'll stick with 'Be What I Already Have'.
Overall though, when it comes to marketing. It is not so much the slogan or tagline that hooks people, but the prescence of adverts themselves and a new slogan or tagline gives a company a reason to saturate the market with new ads without the public feeling that the company is repeating themselves.