I finally decided to upgrade my Mac Mini HTPC (Mac Mini Server mid-2011) to Yosemite. I run the installer, says it's going to install, reboots, does the spinner for a bit, reboots again and I'm back at the login screen in Mavericks. If I login, it's like I never tried to install. No errors, no messages, just ... back in Mavericks.
I downloaded the installer a second time to see if the one I had was corrupted, same result. I scanned the console quickly and while there are messages there, nothing seems like a smoking gun to me.