H2G's fourth and fifth book were depressing because Douglas Adams was, shockingly, depressed. Other than that, the humour got away from him a bit as he had to go ever-more-outlandish and because he wrote them straight to book unlke the earlier three which were first radio/tv/newspaper/live action/whatnots.
GRR has already stated that he has a lot of time to fill, so to say, because he misjudged distances and travel times. Tyrion needed to be with Daenerys earlier and such. He only realised this somewhere around book 4 or 5 when it was too late for him to do much about it (well, except some Deus-ex-Machina). Sam and Tyrion need to unite with Dany before much else can happen that-a-way. Bran needs to grow up a bit because he's too young for some of the stuff that's supposed to happen to him...
Bad timing, I guess. Other than that, the biggest problem, I think, is that he's too busy with everything else.
What I'm afraid of is that, either he'll have a heart attack and die and leave it unfinished, or, he'll "have" to finish the later books more quickly so as to stay ahead of the TV series, and both the story and the storytelling will suffer as a result.