First problem with Mountain Lion, Preview doesn't scan. Although Mountain Lion includes a significant upgrade to Preview, it seems that there are still bugs to iron out. Wireless scanning is finally enabled out of the box and no longer needs to be reenabled every time I reload Preview. The bug I have ran into is that I get the warning "The selected folder is not writable" when I hit "Scan" and the app refuses to proceed. Here is the fix if you have that problem:
1. Close Preview
2. In the Finder, choose "Go To Folder" under the "Go" menu
4. It will open the hidden folder. Delete the file "com.apple.Preview.plist"
That's it! Preview should now scan properly. The only other problem I've had with Mountain Lion is that the printing manager no longer auto-quits. Have you found a solution to that problem?