Don't get me wrong, the last thing I want is New Vegas linearity where it's almost a crime to do off rails exploration. Though I do hope the main quest is set up in such a way where you can attack it from different angles and still have a fulfilling experience.
Like if I stumble upon a mid - late game event early on, the main quest would proceed down an alternative path, allowing me to still experience the story from another angle. Versus, heading to the strip in New Vegas and finding Benny soon as I leave Doc Mitchell. In the event that I do make it to the strip early game, this would trigger Benny catching wind of my unexpected arrival, causing Benny to flee somewhere else and let the story unfold a different way with alternative quests since I didn't head towards Nipton.
Or in Fallout 3, if I headed straight to Vault 87 after leaving 101, this would trigger the Enclave kidnapping my dad and a new set of quests would unfold allowing me to experience Fallout 3 from a different perspective instead of allowing me to automatically purify the water and end the game.
In other words, if I follow the story, one set of events would trigger. If I stumble upon a latter portion of the original quest line, an alternative quest would automatically trigger, ultimately leading to it's own conclusion. So there would be no real way to speed through the story, since alternative stories could pop up depending on how I explore.