Do you ever learn how to do something that, after you've figured it out, instead of being happy with the result you're just annoyed about how long it took before you googled it successfully?
So, WiFi apparently broadcasts on one of 11 different channels. When you have a lot of routers broadcasting on the same channel, it can produce interference -- so if, for example, as was the case in my neighborhood, your router is broadcasting two signals on channel 6, and several neighbors' wifi reaches the house and is on channel 6, your wifi is going to be crap unless you're literally sitting next to the router -- as was the case in my house.
Because apparently Comcast's "Automatic" setting for channel selection actually means "Set goddamn everything to 6."
Today we went from a state of several years: not being able to get a solid connection twenty yards away from the router, to a new state: to having full strength signal everywhere in the house, and the only thing I can manage to feel about it is irritation that I didn't figure out how to do this sooner.