Can't assign keyboard shortcut to service
One possible reason is that your user account has lost rights to change services preferences file (some buggy installers can cause that). Open Terminal and type

ls -la ~/Library/Preferences/pbs.plist


If you see that the owner is root, that is the problem:

-rw-------    1 root  wheel    1407 Jan 16 13:40 /Users/me/Library/Preferences/pbs.plist
                  ↑
                 incorrect!


In that case, you can fix the problem by typing this command (replace me with your user name; you will be asked for your password):

sudo chown me:staff ~/Library/Preferences/pbs.plist