In this early demo (Codename: Chicago), the minimize and maximize buttons have been redesigned, but the close button remains the same, and to the left as before. I wonder where the author got the idea that the [-] button at the top-left was a close icon. It was the "Control Box", a menu icon. AFAIK it's still there, just invisible -- hit alt+space to open it. Disclaimer: I'm currently unable to test that.
Still there in Windows 7. It's visible in some applications (e.g., PowerShell) but not others; ALT+SPACE seems to open it consistently whether it's visible or not.
My hazy memory is that the control box was retained for accessibility and for the many users trained to use it on Windows 3.1.