There's no protagonist, there's tons of filler scenes, there's more exposition than there is story, the plot moves away at a snail's pace and much of it is just fan service. I haven't read the book but I sat with people who had and they kind of understood the film. Kind of. But me? I had no chance.
A really inept script, to begin with.
I liked some details tho. But start with the script, man. Start with the script.
While I can't agree it was bad I do want to thank you for confirming my suspicion that for those who didn't read the book it would be hard to watch - I never recommend it to anyone who hasn't read the book.
Even though I do enjoy watching it, I agree with your other points - it seems it is difficult to turn Discworld books into films and in this one I get the feeling everyone involved assumed the viewer could sort out the jumble of scenes that often made it seem there are several stories going on, none of them related. Even the best bits, and for readers of the book, there are many, are presented in a way that feels disconnected from the overall production (I'm just guessing the best bits were the ones you were able to like before returning to the confusion of the overall story.)
I suspect the only way to make Discworld work for all audiences would be a television series called Discworld where the series can insert the stories and take as many episodes as needed while traveling the Pratchett timeline.