Finally Downloaded and Installed, so Apple Fixed Issue
Apple must have fixed whatever was preventing the download from working here in northern CA. I tried repeatedly over the past 24 hrs, until this afternoon it just worked. I didn't mess around with my DNS settings, or do anything differently.
So far, I've only found one application that doesn't work with Yosemite, an app called Living Earth, which puts the temperature in the Menu bar, and allows you to click on it to see weather details for a bunch of locations. The Menu bar portion works fine, but the Screen Saver portion crashes the application when it starts up. I notified the software developer to be sure they know about the issue. I like this app because it renders a rotating view of the Earth with live cloud cover on a black screen, with text displays of temperatures for various locations in the corners of the Screen Saver. It's pretty neat. I hope they can fix it for Yosemite. I used to use Meteorology, but they've long since stopped updating their app.