I have an alternative take on this problem:
I have just bought a Classic 160 Gb at Amazon few weeks ago and I have been facing the same issues as you.
I do not use Mac OS X, nor iTunes to sync. I run Linux and I use Media Monkey on a Windows VirtualBox Machine.
During the past two weeks I have tried everything that I could find. iTunes have not even been able to read my entire collection (strange, right?)
My findings
I have just restored the iPod firmware a few minutes ago. Started MediaMonkey and sync'd 10 songs from an ordinary random artist. Disconnected and the songs were there, nice. So the iPod is actually working. So far, so good, I have figured that out before.
However, in the meantime, while I was checking my library, I recalled that I have some songs from Japanese bands with Japanese characters in their names. Historically, this would be very error prone. So I did gave it a try, and sync'd 64 songs from that band. Disconnected the iPod and the songs were all gone.
So apparently the 2.0.4 firmware does not handle these characters very well. Check your library, make smaller tests. I am about to restore the iPod again and will try to sync some more files with normal ASCII characters. Will update you.
My old, now dead (waiting for a new disk), iPod, running 1.0.2 was able to hold those files nicely.
Best Answer
Im not sure this is default behavior. I use stars and smart playlists to actually help manage things (smart playlist to delete songs, and then rate things with 5 stars that need to be 'processed'). I sync usually once a week, and it always syncs these changes back and forth.
Its supposed to sync all the songs in recently played to update their metadata (like playcounts) as well. Maybe if you don't sync for months, that playlist doesn't show everything, and therefore looses some of your ratings?