For me it's the ability to view the vast amount of items in the inventory as 3D objects and zoom in/rotate.

Moving bear-traps.

For me, the High Resolution Texture Pack.

And the release of the Construction Kit, so that the wonderful talented people could make mods for the game.

I was looking forward to the in-game economy system that Todd made a big thing of so that was a bit of a let down but I would say that the 3D map was one of my favourite features in game, the novelty has worn off a bit now but at the time of release I loved it.

More so specific to Skyrim. The modding tools have been there since Morrowind.

The mountains.

The past TES games been relatively flat and lacked a certain epic scale because of it.

Yeah, Cyrodiil just felt like England in the middle ages... bland, flat and uninspiring. Skyrim's variety in its environment between each hold has been one of my favourite features of the game, especially after I thought that the entire province was just going to be snow.

The nuances created with combined lighting and weather that make the outdoor world really amazing at times.

That is definitely high on my list as well.

Replayability and the vastness of the game. I've been playing video games since Atari Pong and I've never had a game hold my attention this long. Yea, it's had some bugs; but I feel that content and character customization overcomes the frequent reloads.

The random encounters. I honestly like coming across them still even after thousands of times.

The way the alchemy system shows you known effects, and then when you go into one shows you the various ingredients that have that effect.

The oft-overlooked Storm Call shout. It's a bit unreliable, but epic when it works.

Sprinting. Or is that one not overlooked?

The wildlife and mountains

The Khajiit camp outside Riften, at night.

The glow from the roaring fire, and the Khajiit company, makes it surreal just to sit and relax there.

It's one of the reasons I wish TES would include the ability for our characters to sit.

This is pretty much it for me as well. Love the views of the mountains. Oh, and the sky at night. Whenever I exit Vlindrel Hall at night I love to look up at the night sky - AMAZING!!!

Aren't they outside Whiterun and Windhelm as well?

Coming out of Ustengrav and seeing the aurora was just fantastic. Video games have really come a long way since I was young

Yes, but the camp at Riften is special because it's inside a circle of trees. The light reflected from the trees is spectacular.

For the other camps, I usually just dart inside the large tent, especially when it's raining. Too bad it's immersion breaking to see the rain coming through the tent.

Radiant Quests.

When all the main quest lines are done and all the word walls and mask have been found then what else is there to do?

What's that you say? Bandits have moved into Valtheim Towers again and there's a bounty on their heads? It's about time!

I agree with this. It makes it so the game never really ends, which is a unique feature that most games don't have.

I love stream cone spells though that may not be overlooked>

Good to see there's others that like the new Radiant Quest technology as well.