Hmm..for foreign people it's Æ Ø Å and our other vowels in general really.
For Norwegians it's either the skj/kj/sj/ sounds that are pronounced different but almost the same for lots of words or the rolling R sound probably, it's the one kids learn at the latest stage and some never really get a hold of it, like me

We don't really have any hard sounds, I guess that's why we find it easier to take on other languages most of the time.