Both fallout 3 and NV where not about the story (main story) but more about the journey. I enjoyed both stories as far as they where concerned but both could be finished very fast if you focused only on the story. Meanwhile, if you wandered the wastelands, did side quests and just enjoyed the games, you could spend hundreds of hours on each of them.
I did not feel that way with Skyrim though. I have still never even finished it, and I have tried 3 times already (I always end up getting distracted by another game...and then I restart from scratch the next time I play).
Fallout 3 and NV though both have held my interest through completion at least twice each (3-4 times for fallout 3, only twice for NV). I have high hopes for fallout 4 as well, and if it's bigger, that is great (but only if you have more actual character progression to go along with the extra content...I always get bored once I max my character out).
Honestly I don't need a game that is larger then fallout 3, NV or Skyrim. I need a game that will keep me interested like they did for 100+ hours, and still have enough left for me to do that I can play it again sometime in the future (which both F3 and NV already did). Better is great, but I just want them to hit the mark they have already hit in the past.
The problem is the general dumbing down of video games in the last decade or so. I blame World of Warcraft as many games seem to have decided that imitating that game is the best way to sell a ton of games, even if they are in a whole different genre. Character customization and leveling is often drastically simplified and I'm afraid they will do something similar (in fact, I think I heard rumors about taking alot of the rpg aspects out of the combat....making it more based on aim, and less on stats etc).
That is what I am really afraid of. Story is great, and I would like a good story, good main and side quests etc, but honestly it's the actual combat, and character leveling/customization as you play that keeps me interested. Take that away and I'll be bored in hours, while I could have replayed the same 4 hours worth of actually story 25 times as long as I was actually getting to progress and improve my character along the way.
Once you kill any way for the player to improve themselves, you kill most of their motivation to play. For many of us, if you force us to level up a specific way, with no variation or control, then that has a similar effect.
It's just a bit over 2 months to go till release. This is the first game I have really been waiting for with high hopes since D3 (which dissapointed me greatly). I'm getting up there in years, I have 450 or so games backlogged just in my steam library (not to mention hundreds for game systems I have not had time to play) and I can't find myself that motivated to get new games anymore because I just have so many, of all types, that I have not had a chance to play yet. I joke that I could lock myself in a room with food and water for the rest of my life and never finish the games I already have. If fallout 4 is a dissapointment, I may stop even paying attention to the new games that come out in the future, at least till I play through the many, many RPGs (my favorite type of game) that I have, but have not played yet.