Everything. If the path changes at all (different username) then you either have to re-import everything (losing playlists, counts, ratings etc) or edit XML fil

Everything. If the path changes at all (different username) then you either have to re-import everything (losing playlists, counts, ratings etc) or edit XML files and pray it works. Deauthorizing/authorizing itunes account, just the time required to copy the data, including the fact that windows copy is likely to fail part way through. Likely the library isn't all consolidated in the itunes folder so dealing with hundreds, or thousands of gray !'s (file cannot be located). Copying apps (if they have an IOS device), trying to synch IOS devices to the new computer without wiping them. That's all I can think of right now, but I assure you there's more.