Craftsman HBW0777 garage door opener does not close door - light does not flash
It was the speed sensor. After a lot more searching, I found a bunch of references to this on forums and youtube and such. There is a common issue with all openers of this style where the optical speed/RPM sensor's interruptor cup slides out of alignment for various reasons. This wasn't the case for me - the actual sensor was dead. I just installed the new one from Amazon and the opener now works.