You can't just look at the state. Both Brad Miller and John Barrow represent Democrat-leaning districts despite residing in Republican-leaning states. Neither of them is likely at risk this year, and Vernon Robinson is not generally taken seriously.
I don't think Herseth drew a Republican challenger, amazingly enough.
The others you listed are all vulnerable this year unless the Democrats do extremely well nationwide.
Agree, it's VERY district sensitive. In CA, I remember when they districted the GOP district around USCB, right down the beach, so as to not put the liberal college kiddies in a dominant GOP district. Cong. Bob Lagermarsino said he could not walk his district a high tide ;)