I still consider the 9x OSes to be "real" operating systems, but then, I remember Windows 3.1...
My reasons for changing are less important (IMO) than the reasons I find it hard to go back...
(1) Video capture on NTFS
I don't have to worry about file size limits due to the file system.
(2) Fast user switching (on XP)
Allows you to have a "work" desktop and a "play" desktop without getting the two confused.
(3) Smoother Multitasking.
at least it seems smoother to me
Plus my video card (ATI AIW Radeon 8500DV) was not as well supported under 98SE early on,
and the choice between Windows ME and Windows 2000/XP was pretty easy.