It's much more that Martin has planned big and smaller shifts in the war that in order to happen require him to kill prominent figures.

It's not really for "shock value", let alone random, it's all plot-driven. Where it's a bit unusual is in the fact it's a also a character driven piece and Martin won't shy from giving such figures, and their actions, the full importance they ought to have in the greater events until their "untimely death". It gives an "historical feel" to the whole. It's still engineered/controlled reality, like all fiction, but it's got more of the sudden shifts in the course of events that you find in most historical conflicts. For the most part Martin has done away with the tropes of the "Hero's Journey" that belong with myth building and idealization of figures, an approach that's not so common in epic or heroic fantasy which has tended more to build "historical mythologies" à la Tolkien and in which one of the central purposes is to give the main and often main secondary cast full heroic arcs (and so far he's concentrated all that mostly on Jon and Daenerys). The Robb Starks don't often die in Fantasy because the purpose is to show their growth through trials. In historical fiction the Robb Starks die all the time. Same with the "villains" like Joffrey and Tywin. Fantasy writers kill them off only at the end or only when they have large supplies of greater villains in reserve ready to step up and offer bigger challenges. History offer far less of those life long antagonisms ending in triumph for heroes when they're ready to face those foes...

I don't know how big the reaction to the RW will be, and more to the point on what it will focus (I guess I expect the brutality of it might generate more comments than Robb's and Cat's deaths will surprise and shock as much as Ned's death did. I'm still unsure what they're up to with his wife: is she Tywin's spy as I suspected when she appeared, or was she built up from his wife in the books to provide a confident to Robb and still increase the horror of the RW by killing his pregnant wife too, we'll soon see).

When I read the book the first time I foresaw the betrayal of the Freys coming from afar and it's only the scope/brutality of it that somewhat surprised me (but I was sure Robb and probably his uncle and mother would be murdered there, and I suspected a secret alliance with Tywin). It's heavily foreshadowed in the TV show as well, IMO - the two colleagues of mine who follow the show but haven't read the books are dead sure Robb is about to fall into a trap at the Twin Towers (and all doubts they had vanished when they learn Arya was heading there too...). Again it's more the scope/brutality of what's going to happen they don't see coming, but they sure expect it's a trap. They didn't miss the big clue that if Robb would need the Freys to attack Casterly Rock, the Freys would make a good ally for Tywin too, and they find Tywin suspiciously unfocused on the Starks right now.

I think Tyrion's downfall, the deaths of Joffrey might surprise non-readers more than Robb's death. They really built a feeling of impending doom around Robb this season.