The unevenness of the writing, the hints in other books not followed through on, and especially the ending is a result of SK's preference to forgo plotting. In On Writing, he states that he has plotted out only 1 or 2 books, and doing so makes the experience of writing less enjoyable for him.
Unfortuately, a seven book series, an epic series, needs an overarching plot. The make-it-up-as-you-go method really does not serve the series well, especially with the years-long gaps between some of the books.
I really liked the story in Wolves of the Calla, but the entire thing could have been left out without really affecting the series. That seems like a weakness to me, but it may be a strength for someone else. Also I hated the flashback bit in Wizards and Glass. Not the story, but how it seemed like a cheat (hey, I wanted to hear about these characters, not those), or like when you start out on a long journey, but then have to turn back and make sure you locked the front door- a long detour.
I also really liked Bag of Bones. It's among my favorite King books. It has a tightly woven story, it's creepy and sentimental. I also liked (didn't love) From a Buick 8, more so than my husband who hasn't read the DT series. I liked that there were no real answers- kind of like real life most of the time.