Windows XP was arguably Windows 2000 with a Fisher-Price UI bolted on, which in turn was "just" Windows NT with the Win98SE shell bolted on, so there's that, too.
(My MS kernel developer friends might disagree with this assessment, though)
Windows XP incorporated some QOL features we take (probably?) for granted today from Windows ME into the Windows NT line, like System Restore and Automatic Windows Update.
XP introduced VEH, a more "Unix-like" method of handling exceptions. I recall this was one of the reasons the Golang devs dropped support for 2000 at 1.3x.
(My MS kernel developer friends might disagree with this assessment, though)