Why? Microsoft has a reputation for releasing buggy software. While Windows 7 has been widely tested by real customers, history presents the likelihood that one or more major "gotcha's" will appear.
With previous operating systems, these were usually fixed in a service pack that followed the release of the new OS. Windows Vista, for example, is currently on Service Pack 2 (SP2), while Windows XP is on SP3.
In the case of Windows 7, Microsoft has said the first service pack is likely to be issued about 12 months after introduction, meaning a year from now, in October 2010.
If you are happy with Windows XP and leery of Windows 7, maybe this timing makes sense. Or maybe you'll go ahead and upgrade regardless.
Either way, it is important to realize that Windows service packs are not what they used to be. There was a time when service packs were the primary vehicle for delivering fixes for Microsoft operating systems.
That has changed. Today, Microsoft issues a stream of patches throughout the product's lifetime, with service packs used to "roll-up" all of the fixes into a single piece of software that will install all the patches released to-date. Service Packs may also install new features and upgrade old ones.
Gartner, the big IT consultant, has urged its clients not to wait for SP1 to begin testing Windows 7. Part of this is because Windows XP is already on what Microsoft calls "extended support" which ends in 2014. It is also, I think, recognition that Windows 7 has been thoroughly tested and Microsoft will quickly issue patches as they are required.
Finally, it may also recognize what some have already suggested: The Windows 7 not so much a new OS as it is Windows Vista Service Pack 3.
My take: While I am am not planning to upgrade (Vista) or migrate (XP) my own machines to Windows 7, it is because I am planning to purchase new hardware instead. I am not terribly concerned about showstopping bugs in Windows 7, so I am also not waiting on the Service Pack to begin using the new OS in my home office.