It's not really considered a 1.3 thing, but, support for Macs. I honestly don't feel like pulling out my Windows laptop or Windows Partition to play, and a find that using Steam with Wineskin works, but I can't get Terraria to work.
In terms of actual in-game features, I'd have to say Steam Integration, simply because if it was an April Fool's joke, it would feel cruel, and I don't think the devs would do that. If it is a joke, oh well, and my favorite feature will likely Expert mode.