I finished ADWD last night and my feelings on the whole series are complicated, it's going to take a while to digest.
To jump into the discussion, I think it's fair to say that not a lot happens in AFFC/ADWD, but the truth is that not a lot happens in any of the books. On the whole they're very thin on plot. I think not focusing on plot is totally valid though, you can still have a good story which is why I'll keep defending Mass Effect 2. Of course ASOIAF isn't completely without plot and the last two books failed to close on a strong plot point, unlike the first three books, they just set them up. I just don't think that's grounds to crucify them, if you were just reading for plot I suspect you were already frustrated with the books.
For me a bigger problem is that he (GRRM) fails to make the things he meanders on as interesting as in the earlier books. Arya and Brienne running around the countryside is equally irrelevant to the plot, but Arya is a much stronger character who develops more. Daenerys' character development also grinds to a halt in a comparatively underdeveloped setting. Meereen should've been another King's Landing to justify such a big part of the timeline but it isn't even close. I thought Tyrion started off fine because at least he gets to be himself a little, but the Penny stuff was just ugh. He didn't need to be more empathetic and that wasn't a good way of achieving it.
Jon's arc felt stretched, it's headed in the same way from the start and never diverges. I guess he really wanted to sell the ending. A few other arcs felt that way too. Cercei walks a fairly straight path to her demise that could've been sped up, and most of Jamie's character development actually happened in ASOS. Even though I didn't feel this way when reading it I have to say that the story of what happens in the North as well as in Dorne is what salavages the books in this timeline. That's where we get some character interactions that are actually interesting. I think one of the problems on the whole is that he wrote several of our favourite characters out of the story (temporarily at least), but he still wants to keep them around even though they don't have much to give.
Finally, I think one problem with the books is that they introduce yet more tension into the story without resolving any of it. That tension is almost entirely in the form of the separation of characters. At the outset none of the Stark kids know much or even anything about eachother's fates, Daenerys isn't in Westeros etc. and that doesn't change. Then even more tension is introduced by Brienne trying to find Sansa (good luck) and Tyrion trying to get to Daenerys. You could say that's just the lack of plot, but I don't think it's quite the same. I think that build up of tension is what people are actually frustrated by when they say that nothing happens, but resolving some of it wouldn't necessarily be a big plot point because most of those characters are more important to us (the readers) than to the fictional world they inhabit.