So... Here I was playing sets on my new Traktor S4 on my 2010 iMac, and noticed that Traktor was acting weird--comments were getting erased, ratings were disappearing, play counts weren't being kept. In other words,Traktor's explorer worked on some files and not others.
I tried changing file permissions, reinstalling Traktor, etc... nothing solved the problem... Doing research didn't show me any concrete solutions (e.g. change the play count settings; this didn't work).
Then it occurred to me, I haven't defragmented my Windows PC in over 2 months. And a few weeks before, I had my windows PC sync all my mp3 files to my iMac.
So long story short, the mp3 files were messed up ("garbage at the end of file")...
The solution
Downloaded and ran mp3val for Windows, and downloaded mp3 Scan & Repair for Mac OS X.
It found a bunch of corrupt mp3s--about 40% were invalid. Fixing those corrupt mp3's fixed Traktor's play count, ratings, comments, etc...
BAM! Now waiting to get my Traktor 2 Pro free upgrade this Apr 1st!