Gothic and Gothic 2 had vast worlds that were still fun and interesting to explore.. ON FOOT. I think the key is to make them detailed and non-generic. If I start recognizing patterns in the terrain (Boiling Point), it kind of kills the fun for me. If I explore and explore but never feel it is worth it (Morrowind) I start hating the great wide open. But Gothic/Gothic 2 had that great balance between rewards and boredom. Just when you start feeling bored, a ruined castle (for example) pops up on the horizon. That's how I want it done in Fallout 3. The problem is that you have to hire map designers who are really passionate about their work so the maps don't become fillers.
My dream game would have a huge world and vehicles. Not vehicles like in GTA, where they blew up all the time and you just grab a new one. Vehicles like in.. uhm.. I have no clue, but vehicles that you feel attached to, that you can upgrade as the game progresses (better armour, better tires, stronger engine, bigger gas tank, better headlights, mounted cannon, stuff like that). Of course, in a Fallout universe, the cars would be very rare and part of the exploration rewards could be new parts for the car.. [/dream]