Ed Bott says that his favorite feature is the new rewritten audio engine in Windows Vista (although he focuses on the visual interface). That was done by Steve Ball’s team (which Larry Osterman, one of my favorite developers I met while at Microsoft, is on).
Second favorite feature? The rewritten networking stack. You’ll never see a good review of it, but the networking performance is a lot better. All that plumbing work will pay off bigtime when we all start throwing HD video files around our networks at home.
Third favorite feature? The security enhancements. I hope never to hear that something bad happened cause Windows’ security isn’t up to par.
What’s your favorite feature?