Adding to this:
Season started out well but the people I was with who hadn't read the book had a hard time following some of the jumps. The second book is maybe the weakest of the series right at the beginning, but it does make me wonder about the series' longevity.
Well, there've been too damn many characters to keep track of pretty much from the beginning (the book's got an appendix, the show's got a website), but yeah without Ned as an anchor it gets even harder to keep things focused. Tyrion's as close as the show has to a main character now, and a lot's riding on his shoulders -- but books 2 and 3 really are about his arc more than anyone else's IMO, and Dinklage has proven himself more than up to the task.
Really on the whole I'd say the second and third books have pretty satisfying arcs to them; it's the fourth and fifth where things get to be kind of a mess.
Lack of a "main character" is part of the problem, but really it's a symptom. The major problem is that the disparate characters' arcs connect less and less, and, worse, the pacing of those arcs is staggered.
Minor book 4 and 5 spoilers follow: [spoiler]Shit's happening quickly for Jon, say, but Arya's mostly cooling her heels, and Bran and Tyrion's arcs are mostly taken up with the very long trip from Point A to Point B. Theon -- who I would argue is the main character of Dance -- gets a satisfying character arc, but not a satisfying plot arc; his growth as a person reaches a satisfying resolution, but WE DON'T KNOW WHAT THE FUCK HAPPENS TO HIM; unlike Jon or Dany, he doesn't even get a satisfying cliffhanger.[/spoiler]
So as much as anything I'd say the timeline becomes a problem; the various characters' stories stop syncing up, and it becomes harder and harder to give a given book a satisfying climax and resolution because while one character's doing stuff, another character is cooling his heels somewhere.
(Which reminds me, the third new scene I meant to comment on: Robb and Jaime. Great stuff, great choice, and I hope there's more of it. I think the books made the right call in having Robb as the only non-POV Stark (aside from 2-year-old Rickon) because it's significant that none of the actual Kings are POV characters, but TV is a different thing.)
Back to the pacing issue: Martin knows it's a problem, obviously, which is why he split up the last two books the way he did. But it still doesn't quite work -- I like the last two books all right, but they're not as satisfying as the first three.
TV actually is a better medium for that problem IMO; yes, we're obviously looking at season arcs, but a TV season is generally a lot more episodic than a novel. I have no doubt that they'll integrate books 4 and 5 (provided the show gets that far, and I think it will), and maybe they'll be able to tweak the timeline to intersperse the various stories better.
Maybe. There ARE a lot of maybes here.
Maybe there'll be a sixth book by the time we get to season 5 and the TV writers and directors will still know what's coming. Hard to say. I'd like to think the pressure of the TV show will light a fire under Martin's ass and get him to finish the books faster, but I understand his difficulty -- it's not just that his material is getting even darker as it goes, it's also that the whole thing's turned into something of a jigsaw puzzle he has to fit together.