So, I turned on File Sharing on both machines (Mac is running OS X Mavericks 10.9.1, Windows box is running Windows 8.1). Mac can connect to Windows fine. Windows, however, cannot connect to Mac - it keeps complaining that the credentials supplied are incorrect. Checking the box buried deep within the Sharing preference pane to enable old-style sharing for Windows makes things work, but what doesn't make sense to me is why it won't WITHOUT this option.