Yeah, In open worlds you usually know what you´re going to find (another mountain, another road, another street...). With variations but with a set style. But here you are in an alien planet, everything is foreign and you could find next a huge floating rock, a cave with low level enemies, an alien base with strong mechas, a group of cute rabbit-like creatures chilling around a lake, an enormous dinosaur that´ll just ignore you...You can go from a dense fluorescent jungle to a barren open desert in a moment. Also, the transitions between zones are very well done, many people have said that they first thought they had just found a hidden cave when they first stepped into Noctilum and yesterday I went a few hexagons deep into Primordia on foot from Oblivia without noticing due to the smooth transition.
I was really excited for Zelda U´s open world, but it´s going to take a lot from it to impress me after this.