Lets see.
NTFS-FAT32 Backwards compat/homenetworking.
Java support (non-existant).
Tones of BSODs (especially if you had some of the old SB cards).
Massive security holes that could only be described as planetary body abiding in size.
.NET was a _MESS_
DirectX was buggy for a few months (or maybe it was just one, but it really forked a few of my older games for a while).
Integration with Firewalls (most) was terrible for almost half a year.
Inferior to 2000 (heck, still is).
The list goes on.
Although it's a slightly different economic environment now for comparison:
Windows Vista sales outstrip XP launch | 27 Mar 2007 | ComputerWeekly.com