Just thinking about this the other day, and realized with over 2000 hours of play, there are things I still haven't accomplished in Skyrim on a single play through. Even though I've wanted to. Rping gets in the way!

Here are a few of mine.

1. Collect all the dragon priest masks. Most I ever collected was four I think.
2. Purchase all houses and homesteads. One character had three houses.
3. Collect all the Daedric artifacts. Almost got this one! One character had
eight!
4. Collect all the Stones of Barenziah. My stealth characters usually top out
at around 12 to 15 stones.
5. Never got Enchanting to 100. Not even on my Mage builds. Don't know
why.

I'm such a slacker!

Anyone else have things they've never accomplished?

Never been a vampire

Never done a mage playthrough (I don't like magic all that much)

Never dropped diamonds and made bandits fight over them. I always want to but I never do.

Never turned down Ulfric's request to kill Tullius. Does Ulfric do it instead or is he beheaded later?

Never given Harkon Auriel's Bow.

Never got the achievement for building three homesteads, even though I did it twice. Oh well.

Never assisted the Thalmor in Markarth or on Solstheim.

Never played as a Khajitt or Argonian.

Never partook in canibalism