Same art style =/= same engine
Sure, there's going to be a tiny bit of code reused (I don't think developers throw absolutely all code out the window), but to say it's more than 5-10% Gamebryo would be a step too far. I mean, the resolution of those textures and the draw distance, let alone everything else just seems out of Gamebryo's rendering league.
Since you guys want a summary, I'll write up a quick list.
-Obviously improved draw distance. Also claimed that anywhere you see, you're able to go.
-Dynamic soft shadows and lighting.
-Snow will pile up as it falls, rather than simply being a preset mesh and texture.
-No class system (Fine with me, I used customs all the time anyway).
-For levelling, it's more based on the progression of the skills you enhance. A skill that you raise from 34-35 will be more advantageous to levelling up than a skill from 11-12.
-No HUD in first person.
-Fast travel returns (Again fine with me, either use it or don't).
-Better level scaling than Oblivion (closer to Fallout in that regard).
-Random generated quests return. The game watches where you've been, what skills you've trained, etc, and will create a quest in a new location with something more suitable to your skill. Raising skills (for example Destruction) may lead to a quest also, as someone may be willing to challenge your skills in battle in a duel on the streets.
-More expansive combat. Bind varying weapons/spells to either/both hands and use them simultaneously. Shields can bash for a stagger effect. Longer bowstring drawback, but greatly increased damage. Finishing moves depending on the enemy and your weapon. Sprinting for quick movement at a reduction of stamina, and a slower backpedal rate.
-Much better looking NPCs that won't zoom in on a time stand still basis when you interact with them. They'll move around and perform tasks while talking with you, and will even talk to you from afar.
-Radiant AI is coming as it truly meant to be. Drop a weapon and watch people fight over it, or have someone return it to you depending on their personality.
-Children are confirmed to be in the game so far. That being said, I can't see it not being rated Mature (especially with what happened with Oblivion).
-Lore seems to be much more rich in this game. One of the most anticipated areas (for me anyway) making its way into the game are the 7000 stairs.
-More streamlined UI and menus (they referred to it as adopting an Apple based view on UI).
-18 skill total (a few are likely to be combined. At first I was worried at "dumbing it down" but with the new class system it makes sense. Levelling would take forever otherwise).