First picture, that fold is the lower half of his bicep. Second picture, the veins are normal as having large amounts of muscle mass increase your vascularity.
Indeed, it's nice to see Bethesda putting realistic details into character models, after the completely smooth skin Oblivion's character's had. I want characters that look like real people here, when I want to see anime characters, I'm perfectly content to watch anime.
The only problem I have with those arms is that they look a bit low on the polygon counts, but there's not much one can do about that, I suppose, Bethesda needs to make sure the game runs well on current generation consoles, after all.
While we're at it, anatomically speaking, the fingers are wrong/weird as well.
The hands look fine to me, actually, I'd say they look better than the arms in the first person screenshots, perhaps because their low-polygon counts are less noticable.