I think that GoT has lost a lot of narrative power as a result of multiple "main thread" characters being killed or dying off. I very much appreciate an author who is willing to kill his or her characters. It avoids trite happy endings and strikes fear and real feeling into the heart of readers, who often think "Oh but [the author] would never allow this character to die, they are too integral." It also creates a more realistic story.
However, when you repeatedly kill characters, especially characters that your audience likes, you encourage your audience to stop caring so damn much about your characters in general. The reader (or watcher) learns that everyone is expendable and so as a result stops investing as much in everyone. Ultimately, each main character death becomes less meaningful in the long run as a result, and in order to create the same kind of reaction as (Book 1 Spoiler) Ned Stark's death the author must ramp up the extremity of each death. See (later book - either 4 or 5) the Red Wedding. In addition, due to the narrative structure of GoT, each main-line voice that is killed off must be replaced by new characters, or face a dwindling cast of main-thread characters and PoV. GRRM has obviously chosen against the latter. In fact, the main-thread characters have increased. On top of that, GRRM has at times introduced characters whose narratives only appear once in the story or very sporadically. This last bit causes me to again care less about individual characters because I no longer know who's important or who is coming back. Each new main-line character must cultivate a relationship with the reader; the reader must find either the story, PoV, or the narrator appealing, or else why bother reading that section? When this relationship is hastily cultivated, not cultivated (by only having one instance of it), or not thoroughly cultivated, the reader will lose interest. I feel that GRRM fails to realize this in later books as he begins introducing a great number of new main-line characters who I felt (as a reader) I had no or little established reason to care about. It is trying to continually attempt to establish relationships to new characters/POVs/storylines. GRRM further complicates this as he introduces more and more complex storylines. When you are jumping from place to place and person to person, it is easy for the audience to become confused about who is whom, where a given character is, and which events the character has experienced or is experiencing. This is of course even further complicated by the fact that characters move about all the time. This results in a significant amount of thumbing back to previous chapters so the reader can refresh him- or herself on prior events. As a reader I find this extremely frustrating when it occurs on a heavily repeated basis. I don't think one can deny that it makes it significantly more difficult to progress forward in a book if one is constantly thumbing back to ensure one has the correct understanding of a situation. And in addition, it builds on that detachment that I mention earlier.
Many people have a few favorite cast members, as I'll call them, and they read on primarily for those characters. The attachment to those characters and their fate keeps the reader going. Unfortunately this can also result in readers paying less attention to stories that don't involve those characters, skimming, and thereby not forming the new attachments that are necessary to continue interest in the overall story as a whole. So this contributes to the "not building relationships with new characters" issue - because some characters a reader has deep relationships to are still left.
I think GRRM got overswept with his Grand Brilliant Idea and began throwing in more extraneous detail and story than was necessary. I think his editor got overwhelmed with the success of the books and is basically letting GRRM do whatever the fuck he wants. And I think the text, over time, encourages readers to become alienated. Basically, I think that shit needs to get edited the fuck down.
I realize my complaints may come down to "GRRM/GoT is overly complex" and some people will say "Complexity makes a work great." Sure, yes, complexity can help work be really, really successful, see Infinite Jest. But like any other tool or technique, when it's overdone, it becomes harmful to the text. Some of what I complain about in the long run helped make GoT what it was in the short run/beginning. I guess I feel like GoT is now pulling the same old tricks, same old rabbit out a hat, except now I've seen it ten or fifteen times and because I see it coming, I'm not even interested.
I'd rather read the Wikipedia summaries of every forthcoming GoT book than spend the time it takes to wade through them. The writing and story are not good enough to justify the time or emotional attachment. GRRM isn't doing anything new with the writing. It's plot plot plot and I'm no longer surprised by or interested in most of the plot.