I am very bothered that they changed Robb's decapation of Lord Karstark in the show. In the book it took Robb more than one swing to decapitate him, to which Catelyn noted that "he is still just a boy" or something along those lines. In the books, the ability to decapitate one's enemies in a single blow shows that they have come into their own and that they have developed. Some examples of this are that Theon cannot decapitate Ser Rodrick in one blow and it takes John more than one swing (iirc) to kill Lord Janos Slynt (I have a theory on this as well )
George Martin loves to include subtle themes and concepts into this story which portray the development of his characters and this was one of my favorite ones. Makes meh sads.
BOOK SPOILERS
While I do kinda agree, they did that with Theon already. It would have been quite stale if done a second time. Theon failing at decapitation is still probably quite fresh in peoples mind.
And I think showing Robb ignoring his loved ones and trusted advisors more than showed how "young" they are making him seem. I think they are building up to the Red Wedding quite nicely, by showing how Robb is unraveling as a leader and as a man. Like saying "Oh I know that Frey is an absolute madman, and I made a promise and broke it, but maybe he'll help me"
At the moment Robb is just floundering about from one failure to the next. It's quite obvious that he's lost.