Everyone loves a killer feature: that must-have capability or technology that prompts you to plunk down your hard-earned cash in an effort to upgrade your computing experience.
In the case of Windows, there have been precious few versions that included a truly killer feature. Windows 3.1 was a killer version because it allowed PCs to finally break (or at least reduce the impact of) the dreaded 640K barrier. Windows NT was a killer version (at least for power users) because it introduced the concepts of client/server security and true, hardware-based memory protection to the environment.
[ How to choose between 32-bit Windows 7 and 64-bit Windows 7. | Get InfoWorld's 21-page hands-on look at the new version of Windows, from InfoWorld’s editors and contributors. | Find out what's new, what's wrong, and what's good about Windows 7 in InfoWorld's "Windows 7: The essential guide." ]
Windows XP was a killer version because it bridged the gap between the consumer (Windows 9x) and business (Windows NT) computing spaces. And though generally considered a flop, Vista was a killer version in that it forced the Windows ecosystem to evolve beyond the Windows XP paradigm and thus paved the way for Windows 7.
Which brings me to my main point: Windows 7 is a killer version -- but not for the reasons you think. It's not because it fixes Vista's many faults -- it doesn't. Rather, it glosses them over with fresh paint and behavioral tricks.
It's also not because of the new UI. Although I'm a huge fan of the new task-bar-driven interface, much of the underlying concept is merely a rip-off of the Mac's aging dock metaphor. And it's not because Windows 7 is somehow lighter than Vista -- testing shows it takes up about the same amount of RAM when executing an identical workload.