Windows 10 November Update mysteriously pulled, as concerns about bugs grow

Systems can still be upgraded to the November update, but direct installation is no longer possible. Instead, the original RTM version must be installed, and the upgrade to 1511 performed through Windows Update.
Others are reporting all manner of bugs, such as machines that instantly wake after sleeping, excessive processor usage by the Mail app, the lock screen failing to hide the full desktop, and more besides. Given that Microsoft is still pushing 1511 out through Windows Update, none of these issues would appear to be fatal, showstopper issues, but clearly this release is not without its problems.
The way Windows is developed has changed radically as a result of two things: laying off large numbers of dedicated software testers, and shipping regularly both to members of the Insider program using pre-releases, and to mainstream users with monthly updates and twice-yearly larger upgrades (of which the November release was the first).