Let me put it to you this way: when I walk into work, I'm going to tell my boss that we need to upgrade to Longhorn because it helps his business do what?
That is the real question Microsoft will have to answer. But it's a general problem for the whole industry. Why do we need faster CPUs? Or more broadband? Or yet more features in Word?
Win98 + Office97 goes a long way towards covering the feature needs of the individual. Win2000 + Office 2000 does the same for business. Heck, some people are still happy with Windows NT.
It's not hard to figure out new features. But it's harder to figure out good ones that makes the user warrant an upgrade. The new features have to be embraced by the end user in order to pay off.
So it's a challenge both for the software vendor and the customer that implements the new versions in their organisations.
Right now alot of the momentum is in security and collaboration. Enhanced security is easy to leverage to increase sales. Fear is a powerful emotion.