Microsoft is said to be strongly active in vetting Windows 8 apps, which will be important from a credibility standpoint -- Google and Android suffer from an inactive application security vetting. Importantly, Microsoft gets to control the "experience," which they’ve wanted to do for a long time, but have allowed security vetting (driver signing, for example) to be haphazard at best.
Don't think that Windows 8 on tablets will be your current PC on a tablet; Microsoft won't let tablet sales cannibalize their desktop/notebook sales. You may use Windows 8 on each device, but you probably will learn to shuck your old licenses and apps in favor of new ones. Or at least that's what I think their bet is.