I'm surprised a topic hasn't been made for Fallout 4...
As I'm relatively new to the fallout world, only really playing fallout 3 a bit, for whatever reason I had terrible luck with save files corrupting, more in depth with New Vegas, but really hopped on the Bethesda bandwagon with Skyrim... We'll just skip over the kleptomaniac side of how I approached Skyrim.
With the new ability to break junk down into the components (eg, duck tape down to adhesive) to make an awesome array of guns, weapons, and buildings, the games on the pipboy -- which may take up most of my time -- and the graphics. And anything can be done. That's the cool stuff.
The open world games have always been a favorite of mine, not having to do missions, wandering around chasing tales, blowing stuff up, and as Fallout has always been supportive of the latter, it should be a wonderful way to use my last bit of vacation. It's a world where anything can happen, and I'm so excited that it's coming back in two weeks.