First thing to try if you haven't already is rebooting your iPad.
Something else to try, because sometimes iPads like the rest of us get confused, is to sync just a single song to the iPad. Assuming this works and it should, it'll remove everything that shouldn't be there, sort out it's database and allow you to sync everything else.
Oh, and make sure on the Summary Tab in iTunes, you've selected "Sync only checked songs and videos" and "Manually manage videos" if you haven't already.
TrueBlou, thanks for the ideas, I have rebooted. By removing a couple of items selected for sync transfer, I, then can sync. I don't understand why I can't transfer items to the pad when there is lots of room available.
After you got some things synced across, was the iPad still reporting lots of free space? Use, Settings -> General -> About on the iPad itself just to confirm what iTunes is reporting. For example, mine (in iTunes) reports that I have 13.61GB of free space left. But when checking in the device settings, there's actually only 10.7GB of free space, so don't always trust iTunes implicitly.
If there is still lots of free space, try adding the things you removed from your sync a little at a time, it could still be that there's some confusion in the file system reporting. Normally this does sort itself out after a sync or two.