I have just recently been trying to figure out what makes Skyrim and elder scrolls in particular such an enjoyable experience. I would love to see everyone else's thoughts by the way. The most immediate things that come to mind are the way they play on the players curiosity, the free roam, and rich cannon. I can honestly say that for me it is their originality that makes it special. Morrowind had gigantic bugs that act as taxis for the inhabits, a fossilized enormous crab shell acting as a miniature city, and many more. I don't wish to post spoilers but skyrim definitely has the same wonder. It's an almost Alice in wonderland (or the movie that has tinker bell forget the name).
What do you think?

For me it's alot about exploration.
There's always something interesting around the next corner:
"What's that over there? Oh, it's a cave, I've never been in there, wonder what's inside? Oooh, there's a necromancer in here trying to ressurect her dead husband!" (real dungeon in the SE).

Alot of the places has a little story and that's awesome.

The quests are just an excuse to explore.

Indeed. And they know that. They seem to do a great job of making that a focal point.