This has helped some people.
- Sign out of the App Store (in the Store menu)
- Quit the App Store
- In Finder - Go>Go to folder... (or Apple-shift-G)
- Paste in the following "~/Library/Caches/ "
- It will take you to a folder. Delete com.apple.appstore
- Launch the App Store. Sign in and then reclick the Mountain Lion buy. It won't charge you again.
Two other things that have worked for some people.
Start the download again, the click on the Purchases menu to see if the download is paused. Stop the download and then try again.
Boot into the Safe Mode and try the download.
If you monitor the "More Like This" box (top right), other threads appear. Opening them usually displays other threads.