Geez how can this possibly be an issue?? Don't get me wrong, I love my Droid X and have no desire to get an iphone, but I've never heard of them having a problem syncing their music correctly. I use my phone for music more than anything else, and this is so frustrating. A fix for this HAS to be made.
Well so far a couple of solutions have been given, but instead of talking about the solutions people seem to ignore them and keep complaining.
1. Get another music player, they are in the Market.
2. It probably is something in the tags that need to be fixed. Now I don't know anything about Windows Media Player but there is a way to make adjustments on these with iTunes.
I've loaded most of my music using Double Twist and I've seen some problems with incomplete albums. This is because of the way iTunes has music stored in it's folders and thing's I've done to screw them up. If you're having that kind of issue as well just drop the missing tunes in one at a time.