I'm down to the final quarter of Storm of Swords. About halfway through the book the plot really slowed down. The prose remained highly detailed and interesting, but without a lot of narrative movement. This may be what fans complain about.
Overall, the first 2 books and the first half of Storm had better pacing in terms of plot and much more urgency. It was overall a better experience.
The Bran-Horor-twins arc, post winterfell is the least interesting arc. So little happens, and so slowly. I'm up to the point where they meet Sam. Bran is the least interesting Stark and GRRM has commented that the BRAN POV chapters were the hardest for him to write.
And Bran is supposed to end the show on top...
Also I am starting to sense that it was not right that GRRM kill off two major characters off. I can understand the death of one supporting character
but the other two major characters
were highly developed and could have progressed much further into the plot. By eliminating them GRRM also eliminates his (*and readers') investment in them. But maybe I'm wrong and there was a method behind this.