I think for me it's the fact that there's plenty of everything at every embark. It means that I can embark in a large variety of surface conditions without having to worry about whether there will be magma, ore, and other things available.
The caverns are also amazing. In 40d I enjoyed being able to carve out the entire fort exactly to my own specifications, and so at first I was a bit saddened that I wouldn't be able to do my standard "staircase to the bottom z-level, build out from the central shaft" procedure as easily. But in the end, it just means that I just don't dig as deep before establishing my fort proper, and then, after things are settled a bit, I can dig further down for more riches, magma, water, and other excitement. I love the feeling of "untapped recources" that the caverns present to my playstyle, but I also like that I also have the option to play a more organic "lets build a fort IN the cavern" fort.
Also, as mentioned, the increased control over the military is a godsend (once I figure it all out
), and I like that the healthcare system has been overhauled to make injuries both more and less dangerous/random. I'm sure that there are other nuances like these that I haven't even begun to explore fully, but for now, I'm just enjoying that the game has more to offer, and more to learn. As much as I love the nearly bug-free 40d, I was certainly growing tired of how easy certain parts of it were getting.