The lack of attributes.

Attributes and Perks worked fine in unison in Fallout, why not use that system?

Closely matched by the fact all the races play the same, unlike in Daggerfall and Arena where you couldn't roll a Bosmer Barbarian without paying for it you can't notice the differences between the races.

NPCs.

Their lack of personality.
Their unrealistic interactions.
Their repetitiveness.
Their design stupidity. (why did they suddenly think NPCs speaking to you EVERY time you walk within 4 yards of them was suddenly a good idea if past titles (Oblivion, Fallout 3, New Vegas) never did this and no one ever complained about NPCs not randomly talking to you enough? Why am I thrilled to see that my companions in New Vegas are smart enough to know when they can jump off a ledge safely to follow me, but come Skyrim, they've lost this ability again?)

I could forgive the bland storyline, but I could never forgive that guy that works for Belethor at the General Goods Store.

That's the one for me also. related is the fact you can't tell them to just drop dead and you have to pick up their questline so it just hangs there in your log taunting you with the fact the guild is unsolvable. and unkillible.