Unfortunately I didn't take the time to read up on what people were seeing before clicking on the Install button. If I had, I would have run this command and realized I would need 40 hours for this upgrade to complete (my upgrade restored ~5 files per second):
$ find /usr/local | wc -l
I made two calls to Apple support, but they seemed entirely uninformed and were of little help. After 24 hours of a dialog telling me there was "less than a minute" to finish, I finally gaved up and restored from Time Machine.
The best analysis I read on the issue can be found at On the lambda, which shows that the restore of non-OSX files (e.g. Ruby, Homebrew) from system directories is the primary cause of a very slow upgrade. You might also read Jim Lindley's blog on how to prepare your system to avoid the issue.
So if you plan to go to Yosemite, you might want to prepare yourself for a day trip at the park that could turn into an overnight or two. Or you can just wait for Apple to find and fix the aggregious defect causing this pain, and upgrade later.