It should be possible to sort this out and your Playlists should be in tyour iTunes Library.
Try the suggestions I made previously - first of all, another Sync. If further Syncing doesn't remove the Other and re-install your media correctly, then you will have no option but to use a Restore.
As I said above, if you have been using Sync to manage your iPod, then all Playlists should be in your iTunes Library. Playlists created on an iPod Classic are transferred to the Library at Sync time, but they may be renamed. If you saved a Playlist on the iPod, it should be named then as "New Playlist x" (where x is the next unsed number in the sequence) and appear in iTunes by that name.
If you leave a Playlist on the iPod as an unsaved On-The-Go Playlist, it is renamed as "On-The-Go x" (again, the x is the next unused number in the sequence). Be aware though of two things; this is how it works with my iPod (firmware version 1.1.2 PC) and it may differ slightly in an iPod with a different firmware version, such as the later Classics and secondly, the order of the On-the-Go lists is alphabetical, not numerical. So you get this:
where 10, 12 etc. appear before 2. So in other words, you may have to look out of sequence to find your Playlists.
(Don't get too hung up about the firmware version on a Classic. It's really not important in this situation.)
If you haven't been using Sync to manage your iPod up to now, then a Restore will be the time to start doing so and with 100GB of music, it's the best way to manage the iPod. If you are in this postion, we can explain how to do this - it really is easy and by far the best way to go. Just ignore anyone who tells you otherwise!