Entertainment in any medium -- movies, books, games, whatever -- is about doing two things -- artificially creating tension in the audience, and then releasing that tension in a satisfactory manner. Both require some level of skill and/or talent, but the first is much much easier than the last.
The SONG OF ICE AND FIRE books by George R.R. Martin have, so far, been entirely an exercise in creating tension, and Martin has done it masterfully. But when he says "The tale grew in the telling", I personally think he's kidding himself. Yes, he made up more stuff than he thought he was going to because he's as fascinated with the fictional setting as any of the rest of us... but what he's really doing as he crams more and more and more into the story is, he's putting off the point where he has to segue from one phase (creating tension) to the much harder one (releasing all that tension in a satisfying manner).
With SOIAF, that point is going to come when Daenarys reaches Westeros and deals with whoever is left sitting on the Throne by the time she gets there. That will be the first key step in releasing all this dramatic tension he's built up, and everything else in the book (the battle between Dark and Light, Chaos and Order, Entropic Decay and Structure/Civilization) will necessarily uncoil out of it. And the story has become so complex that this is not something that will simply naturally unfold the way a much simpler narrative (like the books I write) would. It will literally require the hands of a master composer to bring this off... it's like a symphony, and every note is going to have to be perfect.
It's why, I think, it is taking Martin so long to get to that point... a point we all expected to happen at least two huge volumes ago, a point we all assumed would occur in the book called A DANCE WITH DRAGONS. Martin keeps dragging it out, making up new characters and new arcs that are meaningless and go nowhere (can anyone say Quentyn Martell?) because (IMHO) he's afraid to move from one phase to the next. He can't handle his own creation. He hasn't got the chops.
Martin has supposedly told the producers behind his HBO show the general outline of how the story is supposed to come out, in case he dies before the show catches up with the books, or something. And I'm sure he's told them something, just as I'm sure he has some concept of how it all ends. But whatever idea he has, it's nebulous and vague and undetailed and somehow, he has to get his story from where it is now to Over There, and to do that, he's got to weave a thousand different threads together in a way that creates a compelling, convincing, and above all, utterly satisfying narrative climax... and I don't think he can do it. I've read a lot of his stuff, and he's never shown any real capacity for bringing off a satisfying ending. Even if he was "the American Tolkien", as so many keep insisting on calling him, I don't think Tolkien himself could pull this off.
Plus, Martin has no real incentive to finish this thing. Right now, he's getting enormous amounts of attention, this saga has single handedly brought all his other books back into print and made them best sellers, and it's generating huge amounts of coin for him. And until he actually comes out with the final book, he hasn't let anyone down yet... the work cannot be judged. My guess is, he'll die with it incomplete and his publishers will hire someone... perhaps many someones, as with his WILD CARDS books... to finish it up.