I've always loved RPGs and open world sandbox games. They're the ones that unleash gaming's true potential as a really interactive experience, that you directly alter with your own actions. As such, I've spent a lot of time listening to podcasts, reading articles or watching interviews in which developers talk about building these types of non-linear games which require lots of atmosphere.
I don't think anyone can doubt that making a game with good atmosphere isn't one of, if not the, main goal here. For a title that has such a large area to explore, it's got to feel right; magical and fantastical, but no matter the genre or take, all imagined worlds have the same principles to feeling alive and real. Fun and actual gameplay comes in later, but if you're spending 300 hours in an unconvincing land, then it won't be fun; you'll always feel a sense of 'this isn't right', and you won't feel immersed, which is vital for enjoyment and emotional connection.
Graphics, obviously, help this. And Skyrim looks to have no lack of beauty. But it's the wealth of small, often minor features or mechanics that truly make or break a game world. Take S.T.A.L.K.E.R., for example. While the textures may be bland and outdated, the lighting is superb. And when combined with small things, it has a huge effect. Why do players get so scared when, say, coming up to a Bloodsvcker layer? Not just because of the streaks of intermittent lightning; it's the fact that, at night, you've wondered past the layer and seen odd lights and heard strange noises. You've noticed dead bodies of blood around the place, which makes the place feel like it's got history. Bad history. You've heard rumours fro, NPCs about this place, and you know what it's about, but not what to expect. Combined with the graphics and the visual clues, they various little hints and details add to making the whole place just feel so...tangible.
After playing these types of RPGs/Open world games for a long time, I've come up with my own little maxim that I think you can apply quite well to all those games that have truly outstanding atmosphere, truly feel real, and truly engage with the player. Basically, after you've turned off the game, imagine the game world. And simply ask yourself: Is it possible for that world to function without me?
Example: GTA. Not an RPG, but still has the open world idea. Take Vice City or San Andreas. Those worlds only functioned with the player inside them. If you imagine that you weren't playing, but the game was still running somehow, it would be utterly dead. Cars would still go round, but nothing else would happen; all those missions are simply waiting for you. The cops wouldn't have to chase anyone down. The gang wars would never happen, as in the game it takes the player to press a button saying 'Start a gang war'. NPCs would wonder aimlessly around, without purpose, and it just wouldn't feel real. The game world is doing nothing but waiting and reacting for one person; you.
GTA IV, on the other hand, pretty much nails it. Thanks to what you've seen or experienced in the game, you know random events can occur. So, if you imagined Niko Bellic went away for a while and Liberty city was left to it's own devices, it would actually make for a pretty convincing world. The police would still chase criminals, as you've witnessed while playing the game police whiz past chasing someone else, so you feel like 'Hey, there's stuff going on that isn't player-dependant'. NPCs would carry shopping home, dash between awnings in the rain, and drive places. Helicopter tours would fly around the city, day would turn to night, there'd be a shoot-out somewhere, the weather would change, NPCs, react, and so on and so forth. It'd be a slightly boring place, sure, but you still feel like things would happen, events would occur, and there'd be some purpose and order without you. The game, on this occasion, isn't simply waiting for you to affect it; it's a realistic, alive place, that you just happen to have a very active role in.
This can be applied across many of these types of games. In S.T.A.L.K.E.R, Duty and Freedom patrols occasionally run into each other and have a fight in one of the areas. That made it feel alive, real, ever changing, and so on. As you went further in the game, you went to places with worse atmosphere. These sorts of events got fewer, until essentially it was only you doing the unique, world-chaining things; the game was just waiting for you the imitate a mission to go somewhere and so something. If you just didn't play, nothing would happen.
For Skyrim, I actually have great faith in Bethesda because despite engine limitations, Fallout did a pretty good job of this. You'd find people fighting over a fridge, or someone with a bomb strapped to them, and these make the place feel dynamic, and like it would be a pretty interesting, eventful place without you (though, of course, not as eventual as if you were there). But really small things could make or break the world of Skyrim. Imagine, for instance, you go to a town and see Jimmy. You have a chat, and he says he's going to start work soon, or really wants to be an adventurer. You go off and do missions and explore the world, but eventually you cross the same town again. Jimmy's there, with a lovely new piece of armour on. He tells you he did a bit of work for a friend, and earned enough to buy this really awesome briastplate. And you, as a player, are thinking "Wow! While I was off doing my own things, Jimmy was actually off doing his own things, independent of me! It feels like this is a real place where people do new things and have experiences regardless of whether I'm here! I can't wait to see what else has changed!". Or probably just think "Wow, that's kinda awesome". Point is, the world now feels alive, real, atmospheric, engaging, and full of wonder and change and dynamicness (I made that word up).
There are loads of little ways to do this. A character gets a new haircut (no, seriously, thing about how amazing it's be to see a character walk about with a new haircut, as if he's decided, of his own will, that it's too long and needs cutting). A house has a broken roof, and after a while, you go back to see the house and it's fixed. A store talks about getting new stock soon, and you come back later to see they've actually got it. You could even have mini-missions going on that you don't take part in. You see a wife running down the street of the town, asking if anyone has seen her husband. You can't take it as a mission or do anything to alter the events, but you find her later, hugging her rescued husband. And you think, "Hey...
Is it possible for that world to function without me."
Sorry for such a long post, and I apologize for any typos or mistakes, and looking back over this please don't steal it as I might use it for an actual article on http://www.gamersguidetolife.com/ 'coz I've accidentally got carried away and written a [censored] novel.. But I hope what I've said has made some sense.