When I mentioned this to Swen when we were playing over at Larian's, he said it's not their intention to highlight *everything* because the screen would be overloaded with labels, as you can interact with almost everything anyway. Also they didn't want to give away locations of items you should look for a bit, and having everything highlighted except that one weird item is also a bit of a giveaway. Or having everything highlighted except that object which is semi-hidden can be quite frustrating as well.
Still, I did have a first initial response like "hey why is not everything highlighted?", and I'm not sure what the best way is to tackle this.