Windows 8.1 Does Not Go To Sleep

I bought a refurbished ASUS UX31a Ultrabook with Windows 8 preinstalled, then upgraded it to Windows 8.1. I don't remember testing it under Windows 8, but at least under 8.1 the laptop would not go to sleep automatically if Wi-Fi was enabled.

After trying dozens of suggestions from all around the Internet (to name a few: disable IPv6, disable homegroups, update Wi-Fi drivers, add wake exceptions with powercfg, etc. etc.), it turns out the solution was to wait for Microsoft to fix it. See http://support.microsoft.com/kb/2921066