Window 3.0 was revolutionary, becausr of it's graphics and sound capability.
Windows NT 3.1 was revolutionary, because it was the first pure 32-bit windows. I don't think it was the first 32-bit OS (the oldest I know of is Linux, which came out 2 years earlier)
Windows 95 was revolutionary because of it's easy-to-navigate UI and plug-and-play support.
Windows 98 was the first to offer USB support and the FAT32 filesystem, but it's not revolutionary.
Windows 2000 offered improved stability and ease of use in a server environment, finally competing with *nix systems. Still not revolutionary.
Windows XP offers an XML-based UI. Not exactly revolutionary.
I don't know much of anything about Windows .NET, but from what I've heard, THAT will be revolutinary.