Microsoft's Steven Sinofsky reveals a problem of Windows 8; it sometimes boots too fast too interrupt. Full details at the Building Windows 8 blog.
If the entire length of boot passes in just seven seconds, the individual portions that comprise the boot sequence go by almost too quickly to notice (much less, interrupt). Most of the decisions about what will happen in boot are over in the first 2-3 seconds – after that, booting is just about getting to Windows as quickly as possible. These 2-3 seconds include the time allowed for firmware initialization and POST (< 2 seconds), and the time allowed for the Windows boot manager to detect an alternate boot path (< 200 milliseconds on some systems). These times will continue to shrink, and even now they no longer allow enough time to interrupt boot as you could in the past.
On the Windows team, we felt the impact of this change first, and perhaps most painfully, with our own F8 behavior. In previous versions of Windows (as far back as Windows 95), you could press F8 at the beginning of boot to access an advanced boot options menu. This is where you’d find useful options such as Safe Mode and “Disable driver signing.” I personally remember using them when I upgraded my first PC from Windows 3.1 to Windows 95. F8 helped me quickly resolve an upgrade issue and get started using Windows 95.