these are all the things i can remember seeing being mentioned:
-18 skills as opposed to oblivion's 21 and morrowind's 27
-mysticism is gone, but its spells may live on in other schools
-enchant skill is back, apparently
-attributes are gone? [i heard several people mention this, so i figured i'd list it]
-HUD is optional? [another thing i've heard mentioned alot]
-restricted class system is gone? [mentioned from magazine, what does this mean?]
-smithing, woodcutting, cooking, farming, and mining are all things the player can do now
-there is a "smithing" skill, it seems
-dual-wielding is now possible
-it is possible to learn how to perform Thu'um at some point [again, from what i've heard]
-enemies, or atleast certain ones, will start dialogue with you [yet another thing i've heard talked about]
-dragons are regular threats and will appear in the middle of battle, and even attack towns, it seems
-werewolves are back, from what i hear
-giant spiders are mentioned as an enemy in skyrim
-level-scaling is similar to fallout 3's, not oblivion's [according to Gstaff]
-perks will be in, similar to fallout 3, from what i hear
-we will be playing as the dovahkiin/dragonborn
i will continue to update this list as more info comes to my attention, hopefully i make finding what we know so far a bit more convenient for people.