I'm almost finished reading SNUFF for the 5th time, and I have to say - it gets better every time.
I agree that it's not the best DW novel, but that's only my humble opinion. I've read terrible reviews of Night Watch and Monstrous Regiment, and they are 2 of my personal favourites. Snuff is a continuation of a theme. Vimes started off in the gutter, literally, in GG. An alcoholic, no-hoper, who solved the crime, found a good woman, found himself, and earned the respect of Vetinari. He has grown, matured and developed over the course of the books, until, in NW and Thud he suddenly has more to look after than himself and the city - firstly his expectant wife and the life they have, and then, more importantly, his son. At the end of NW he tells Vetinari that there is nothing, NOTHING, he can offer him. And he's right. Vimes has everything - more than he ever dreamed of having, and in THUD, the Deep-Downers try to take that away. They fail (naturally), and that's that storyline done and dusted. So what's next for Vimes? Vimes the drunk is now Vimes the wealthy landowner. A story where Sybil or young Sam in danger being the main plot would be re-hashing of older books, so TP turns to another theme he has used previously - racism.
As with Dwarves, Trolls, Vampires and Golems, there are constant references about their behaviour - they can't be trusted, they're filthy, they eat people/children/babies. Then, each one of these rumours/habits/customs is examined and clarified, and you very quickly see that it's the typical White Trash story - everyone needs someone to look down upon, and there's nothing lower than a Goblin. In UA, it was Orcs, but Orcs were too fierce, whereas Goblins are pitiful creatures, who actually believe the things humans say about them. And Vimes, who originally considered himself not to be racist/speciest, ie he hates them all equally, is again shown to be someone who doesn't care about race or species, and only sees the downtrodden and those who tread on them. Someone wrote way back at the start of this thread, something like - why be disappointed when the story sticks to the formula. They wrote that in relation to it being a mystery novel, but the same is true of it being a Vimes story. Vimes couldn't, and shouldn't, act any other way.
That said, I do have my own criticisms of the book. No Death (what!?); not enough Vetinari, and not enough indication that he is aware of what is going on re Goblin-smuggling; Stinky's role; Ach, there are quite a lot of things, but overall, I think it's a great story, and well worth a second, third, fourth and even fifth read!