Thief of Time is very much one of those Discworld books where Pratchett came up with a killer central idea and then never really developed much follow-through. In his best books, Pratchett would develop a great idea which characters and more ideas and themes would develop organically around but, once in a very rare while, this creative alchemy would not take place and the book that he ended with was just okay. There's a neat idea, there's some funny gags, but the spirit and energy of the best Discworld books is wholly missing.
This is the issue with Thief of Time. It doesn't help that Pratchett is taking on a fairly cerebral idea here - of how time itself works and how people messing around with it can cause problems - but trying to explore it in the context of his Discworld comic fantasy series is not a comfortable fit. We've seen a lot of the gags about the Four Horsemen before (in Sourcery) and the Auditors of Reality are among Pratchett's dullest bad guys (even if he does here come up with a way of giving them more character). Novel co-protagonists Jeremy and Lobsang are somewhat undercooked, and even the usually-magnificent Susan Sto Helit (here in her swansong as a major, and underused, character) and her superb can't-be-dealing-with-the-world snark is absent for vast stretches of the book.
It's not all bad, and by this point Pratchett had developed to the point where he could turn almost anything into an amusing read. There's some nice jokes and the idea of a Fifth Horseman who left the group before they came famous is quite well-played. Nanny Ogg also gets a series of enjoyable cameos, a bit oddly, given that most of the Ankh-Morpork Regulars are missing from this novel when most of it is set in the city.
But the novel mostly feels a bit autopiloted onto the page. The pacing is quite poor - this is a 300-page book at best stretched out to closer to 500 - the more subtle character and thematic points Pratchett is making in other novels around this time aren't really there and an apparent romantic subplot that is supposed to be developing is just absent to a lack of chemistry by the characters, to the point it's genuinely weird that it comes up on the closing page.
Thief of Time (***) isn't the weakest Discworld novel and it has some excellent ideas and a few good gags. But in terms of character and story, it's well below-par for Pratchett in this middle part of the series when he was otherwise producing some extremely good books. The novel is available now in the UK and USA.