Well....let's see. First, you have the big, bad villain, the Crimson King. Apparently, although he's supposed to be superpowerful and evil, he somehow locks himself out of the dark tower on some upstairs balcony. And when Roland comes along, the King's weapon of choice? Harry Potter "sneetches." (I kid you not. Good thing he had a boxful when he locked himself out of the Tower!) Oooooooooooooooooooo. And what is the best that he can scream at Roland? "EEEEEEEEEEEEEEE!", like some girlyman. Disappointment #1.
#2. Stephen King HIMSELF tells you, right in the damn book, in the middle of the story, that you won't like the ending, and something to the effect that you might as well stop reading NOW. Never a good sign. If only I'd listened.
#3. Roland's trip up the Tower. In other King works, the Tower is supposed to be more mystical. In "Insomnia", King alludes to the Tower as having different levels of existence/reality. When Roland goes up the Tower...wow, scenes from Roland's past...that we've already seen. Bo-ring.
#4. The "Bobby Wakes Up From The (Season Long) Dream" episode of Dallas (the old TV show). Roland finally reaches the top of the Tower, and the big climax is....he goes right back to the very beginning of the story, in order to live it all over again.
Now, granted, I'm not Stephen King. And I know writing's hard work. But come on. This is the end to the build up of 7 books? I gave every damn one of them away.