About the only thing I'd consider Windows 8 for (non-phone / tablet) would be a Mame Arcade machine with a Mini ITX board and a small SLC SSD. One wouldn't need a "Mame front end"... just make shortcut tiles for each of the games (unless you have way more than 150 or so ROM) . and the hybrid sleep / startup would be pretty much an "instant on".
Other than that... I can't think of anything I'd use the Metro interface for on a PC desktop.
That said... you couldn't pay me to run Windows 8 as a normal Desktop OS without including the hacks to disable Metro.