Man, the amount of variety in caves/dungeons/forts/etc. is such a breath of fresh air after the cookie-cutter dungeons in Oblivion. Just entered a cave that turned out to be basically an indoor wildlife preserve - bears, wolves hunting deer, rabbits, trees, sky overhead.
On a related note, have I mentioned how useful summon flame atronach is? Especailly the part where they explode when they die.