Aside from obvious things like the Pokemon I think what really stuck out to me was the design of the whole region. I really liked how you had cities that really took advantage of the areas they were in, such as how Fortree City was literally built as treehouses and how Sootopolis City was layered inside of a crater leftover from an undersea volcano.
It also helped that the region was huge and had all the land + big water routes that really gave you that adventure feeling, along with the weather. I wish we could go back to having water routes play a bigger role, Gen V really upset me in that they were almost non-existent.