When there's a major Windows release, users and companies face two major problems: software compatibility, and hardware compatibility.
The learning curve for upgrading to a new Windows environment has never been steep, except from the transition between Windows 3.x to Windows 95. However, hardware upgrades have always been a major determining factor on how fast the upgrade pick-up is on a new Windows build. Driver support and compatibility play critical roles. This is one of the primary reasons why large businesses take a long time to do OS migrations.
Then there's the software aspect. Users have to wait for software vendors to retool or update applications. If you've ever had your favorite app broken by a new Windows install--or even an update--you know it can be pain.
With Windows Vista however, Microsoft clearly created a divide. Many users chose to stay on Windows XP while some were early adopters and quickly changed to Vista. Either way, Windows 7 must now try to support both an upgrade from Windows XP and Windows Vista. Thanks to city_zen for today's QOTD suggestion.
The question of the day is: Are you upgrading to Windows 7 from Vista or from XP?
Maybe you don't want to upgrade at all.