When was the last time Microsoft released an upgrade that got you really excited? An upgrade you wanted because it did something new that you actually needed done? I'm not talking about a security fix or a patch necessary to make something work properly (which would seem to include Windows XP SP2), but something that gave you new functionality so important that you just had to have it.
In my life, I remember wanting Office 98 to get long file names, or was that Office 95? Office XP tried to make interesting but little-used features easier to find, a strategy reversed by Office 2003, where a nice upgrade to Outlook became the star. None of these set the world afire.
If you are on an unfinished platform like the Tablet PC, then every update is important, but that's because you are using a work that's truly still in progress. Windows XP was a worthwhile upgrade, as was Windows 98 Second Edition, but XP really required new hardware. These days it seems OS upgrades are much more interesting than application upgrades.
News source: eWeek