This happened to me yesterday. I have a MacBook Air and after downloading and installing Yosemite, it was stuck on "Setting up your Mac" all day.
I eventually got in touch with Apple Support and resolved my issue in about ONE MINUTE. Here's how:
1. Power down the computer.
2. Plug the computer into a power source.
3. Press and hold the following three keys simultaneously: Shift + Control + Option.
4. While still holding Shift + Control + Option, press and release the power button.
5. Release Shift + Control + Option.
6. Power on the computer.
My computer booted to Yosemite and I've not had a problem since.