Just look at how much better 1, 2 and New Vegas are when compared to 3. It's the writing.
None of those were better than 3. 1 was severely lacking in exploration. Huge map with just a few small locations. I haven't played much of 2 because it's that shitty and judging by the extensive commentary available on the codex it's decline from 1 in almost every possible way (including story). NV also sacrificed exploration for "better writing", which resulted in an overall shittier game if you're not a degenerate storyfag. The writing in all the fallouts (except 3 which was utter shit shit) was mediocre at best and only stands out in the way of reactivity. But reactivity in and of itself isn't enough to make an interesting game. The combat has been sub-par in all of them, though the over the top brutality still makes it somewhat enjoyable. The thing that puts Fallout 3 on top for me is that there's a shitton of ruins to explore and they nailed a proper post-apocalyptic vibe better than NV as long as you don't talk to anybody.
At the end of the day the characters and story are dressing. I don't lock myself in a basement and play games so I can talk to people, and while I don't mind a good story and interesting characters, if I have to choose (and it seems like I always have to choose) I'll take good gameplay over good dialogue any day. Bethesda cannot into dialogue but they used to be able to into decent gameplay with lots of room for player creativity and after being mentally sodomized by Skyrim it's nice to see them returning to their roots.
