Everything, but here are my top things:
- Screen aspect ratio being 16:9 makes all my videos play without any black bards or scaling
- The form factor. It's narrower than the Lumia 920 (my other option before buying) and it's easier to use with just one hand
- The body of the phone is gorgeous and feels even better
I didn't care about Beats Audio, but after buying a very good pair pair headphones I can say it does a lot for the quality, not just by amplifying it.