Until I discovered the hard way that files shared across accounts CANNOT be synchronized to the local hard drive... It was a "What the fuck???" moment, surely MS cannot roll out without this critical functionality... Yet, it's correct, you can only sync to the local hard drive stuff that belongs to your account. If it's shared from someone else's account then you can only see the files online and need to download them if you need it locally.
Big fail, and a showstopper for me. Dropbox does this just fine, BTW. so I am reluctant Dropbox user.