i think duncan just assumed that since they were human that we would relate with them automatically.
granted, he was entirely justified in having to make the orcs not look completely evil. evil demon worshiping orcs doesnt work if you want to eventually make them good in a film franchise without it looking like you jumped the shark so best to get that in asap.
i mainly play alliance, so i knew exactly why id want to root for the alliance because i inherently know more about the faction then the audience does.
without that though... they really do look like massive dicks, theyre only saving grace is the queen being kind to garona.
i could very easily see how someone who doesnt play wow could see gul'dan as the only really evil force in the film.
i really hope, that at the very least we see teron gore in the sequel.