The long-winded thing. Many's the time I've heard people discuss the ways in which the latter books of the series amounted to storm and strife that resulted in no meaningful development in the plot. Instead of reigning in a tendency to bloat that makes Stephen King look like Raymond Carver, he indulged it literally up to the point where he died before finishing.

I know you're decidedly against the concept of reader-writer contract (or at least, against fannish demands), but I think that if one is presenting a tale in the traditional manner one has a moral obligation to finish it, simply by the nature of stories being what they are. Whatever the ending is, you have to point at it and say "the end," even if Luke is thirty meters from the exhaust port and we don't know if the photon torpedo is going in. RJ did his best, as far as handing on the duty of completion to Brandon Sanderson, but that he himself did not do it... It's a problem.