Many aspects for me; I can't choose what I liked the most:
-Characters can now sprint by expending stamina.
-Serious dragon combat as dragonborn.
-You can drop a weapon and someone might pick it up and tell you that you droppped it and if you wanted it, drop it and it may lie there in the mud over time, also 2 men might fight over it to see who gets to take it home.
-Entering a city a woman might appraoch you and tell you that her child was kidnapped and will ask you to save him/her.
-"Your version of Skyrim will be built to your specific playing style."
-Ice Wraiths, I thought they looked awesome, very unique enemies.
-Radiant storytelling to where the quests you do is specific to who you are and what you've done.
-"The game watche everything you do. What skills have you improved? What places have you visited? Who have you killed? What weapons do you own? Which figures in the world are your friends? Who are your enemies?"
-Conversations are no longer in a zoomed-in fashion. "Approach an NPC, and a converse will pop up. As they speak to you, the AI-controlled character will move about, occasionally looking at you, or turn away to continue a task like woodcutting or minding a counter."
-Much more voice acting.
-More activities to do in cities such as forging a sword. Makes me wonder if it will be something like Dark Messiah's forging. Also mining, cooking, or woodworking to get away from life-and-death adventuring.
-The menu is just too awesome. It has 4 pointed compass, Skills, Inventory, Map, and Magic. "Push up to skills and your eyes look up to the skies above, where constellations above Skyrim skimmer above Skyrim" "Press right and reach into your inventory Weapons and Armor are organized, and any one of them can be marked as a favorite" "Books appear three-dimensional" Push left for spells, 85 total. Push down and your character looks at a map