Number one for me is the way the game feels when you're playing, when you're fully immersed in it. Sailing at night during a storm, hearing a real storm due to the absolutely spot on sound design. Looking up at Yggdrasil and the stars at night. Seeing the sun shine through the trees in the Black Forest. The constant noises in the swamp and the overall feel of it. The feel of potential danger when going into new areas or when several higher tier enemies show up when you're not fully prepared.

Other things? The easy repairs that cost no resources but require the proper stations. How ships sail in a fairly realistic manner. How that first boss fight felt the very first time you summoned it, and how great that particular boss looks. Discovering that the game gives you just enough direction, but also leaves you to figure a lot of things out on your own which leads to a sense of satisfaction when you actually learn something new, improve your skill (not your character's skill), and as you progress to the point where your character feels significantly more powerful than the hungry stick wielding being that they were when you were first dropped into the 10th realm.

And, of course, whatever is planned for the remaining biomes.