My top ten list:
1) Writing. The story (or stories, rather) is believable and actually makes sense. In some ways it manages to be fantastic but in a subtle way. Other than the obvious laughs, I never actually stopped and said "now wait just a minute, that's just way too extreme".
2) Characters. Some of them are so deep it triggers something in me. First playthrough I set out to do things based on archetypes and my impressions with them. The people I met made me change my mind, and yet the ending was so sad (Cass got killed) I shouted "Damn it" and hit "New game" rather than "Exit game". I didn't expect the game to "remind me of my actions" the way it did, and I loved it for it. The companions and their stories are awesome (I've only done 3 so far though).
3) hardcoe mode combined with forced choices of freedom. Pretty much everything positive has a matched negative, forcing you to make choices. I may do what I want, but there is no such thing as a free ride.
4) Iron sights and the fact that it is one of the few CRPGs that actually *have* 1st person. I simply do not understand game developers forcing 3rd person, it kills immersiveness completely and I think I'm not the only one they completely lock out of the game by doing it.
5) All the "well thought out small stuff that you might not even notice". Somehow I missed how Novac got it's name in the first playthrough, and when I saw it in the second one I was like "Omg, that's just brilliant"
6) The humor and all those obscure references to the 50s science fiction, the "style of Fallout" (only played FO3 and FONV though). It's very consistent, and very well implemented.
7) The slight, but not excessively overdone, focus on survivalism when not using fast travel. I hope the next game though brings a restored railway system as a travel system, and bicycles to get around. Map should be quite a bit bigger though to compensate for increased travel speeds and to make it appear less condensed.
8) I also love the voice acting. The anonymous NPCs could have been more varied, and maybe more voices thrown, but generally I'm still very happy with it compared to other games (TES games comes to mind). We're thrown into this uber gritty world of hatred and despair with little hope to stand on and everything just feels so depressed. Then Victor (William Sadler) comes along and pumps up hope with his "cheerful and highly optimistic view on the world". I mean, I don't think a better choice of voice even exists
Everyone does a very good job, but the voice of Victor is just beyond awesomeness
9) The faction system and how reputations work. The fact that you can wear disguises.
10) Multiple endings and multiple routes to success (and failures). Great value for replayability.