These fields (and probably others) are not synchronized: Calendar event "Location" and "URL"; Address book "IM", "Web Site", "Birthday", "Picture", "Middle Name" and "Nickname". Most of these fields actually exist in both places, they're just not sychronized, so there's really no excuse.
I also whined about this on the Apple feedback page, but I don't have any reason to believe that's anything but a black hole. (Is it?)
<lj-cut text='"Dear Linux People..."'>
---------------------------"Dear Linux People..." I am, incidentally, gleefully awaiting the first doofus who comes here and says, "If you were running the One True OS you could write it yourself, you whiner!", then I'll ask, "Do the Gnome or KDE conduits handle this right?" and they'll say "No, but that's not the point!" and I'll say "Choke on my fuck!" and then someone will post it on Slashdork. So let's not, 'k?
Update, Nov 2005: The new version of Missing Sync is out now, and comes with its own conduit that fixes most of these problems.