All Google integration such as sync is omitted from the Chromium builds and a couple of other features.. for example on Windows (not sure about OS X) the default in browser PDF reader for Chrome is actually a compiled in closed source tool that uses code mainly licensed from FoxIt Reader. I'm sure there's more but those are the most notable examples that I can think of off the top of my head.
sync is actually part of Chromium[1][2]. The big parts are probably the PDF viewer, as you said, and Flash. Chromium also ships with fewer codecs than Chrome (e.g. no h264), but I'm not sure if they're from closed source or just not licensed for redistribution. There's probably other parts someone can chime in with.
