I'm not sure it's a particularly Danish thing but as I looked through my messages to my friends, I realised that I use the
emoji a lot more with my DK friends than anyone else. It doesn't mean love but is synonymous with 'You are such a nice person', 'I appreciate you', 'thanks for trying', etc. My bf does it, too, even to his coworkers - perhaps this is a Danish thing? None of my non-Danish friends do this and one of them said a guy sending a heart to another guy would definitely feel awkward.
Another one is Dannebrog, the Danish flag. If you see a
it is very rarely a sign of national pride (unless the sender is a football fan - they fall outside any sort of category >_>). It is almost always used as a celebration of a person's birthday since having loads of flags is how we decorate for that. If you pass a house with Danish flags lining the driveway, you can be 100% sure it's someone's birthday