Responding more or less to Pooh's points and subsequent comments--
Humor - This is probably the most serious of Pratchett's books to theat time, and since I find his "laugh a minute" books less than satisfactory, I was pleased. He does try to insert a humorous thread, I think, with the Colon/Nobby and Angua/Sally/Cheery/Tawnee story lines. And I think that's pretty much the worst part of the book. The caricature of the posh museum director is funny, and I must admit that the comment about it being "art" if it's got cherubs and urns is worth a small chuckle. The idea of Nobby and Tawnee as a "couple" is almost too unbelievable to be funny--but I think that's what Pratchett was trying to do. The "girl's night out" section of the book that drags, and isn't funny (or fun). Letting Nobby off (since it seems clear to me that Tawnee will dump him) because he no longer wants her because "she can't cook" is supposed to be funny but is too stereotypical. But then, I don't think that Pratchett has been trying to write "funny" books for a long time.
b) As to the charge of "commercialism" -- I suspect the publication of the book and the board game is more due to the publishers than to Terry. DW fans will, and do, buy most anything. There is no need to buy the game or Where's My Cow ?as far as Thud! goes. The game is described and explained, and all of Where's My Cow? is quoted at one place or another. I can't understand how you can miss the point of Sam's reading to young Sam and the whole Thud! game/ Koom Valley war as major parts of the major themes of the book.
I thought I'd explained this well enough earlier, but obviously not. It seems quire clear to me that Terry is satirizing the mind-set which uses "religion" as a justification for war. It's pretty clear at the beginning of the book when Sam describes what Hamcrusher has been preaching--
"He preached the superiority of dwarf over troll, and that the duty of every dwarf was to follow in the footsteps of their forefathers and remove trollkind from the face of the world. It was written in some holy book, apparently, so that made it okay, and probably compulsory. [emphasis added]
Young dwarfs listened to him because he talked about history and destiny and all the other words that always got trotted out to put a gloss on slaughter. "
The beauty of this satire is that it fits on fundamentalists of whatever stripe precisely because it's told as a war between Dwarfs and Trolls. And it illustrates why Hamcrusher is "accidentally" killed by the others and why they must destroy the proof that Tak created the Trolls as well as the Dwarfs. If this isn't a religious war--never mind what the dwarfs sayabout not having a religion--I can't imagine what else it is.
Wars are fought, almost always, with some "religious" or "moral" imperative--consider the Crusades, the Moorish invasion of Europe, the Aryan theories of the Nazi's used to justify the slaughter of non-Aryans, and even the abolitionist part of the Civil War. That's because war is irrational. Faith is, by definition, not rational. And religious coloring can be added to whatever war or atrocity you want to justify. The most obvious analogy is to the Al-Qaeda terrorists, but it works just as well with the anti-abortionist murders or other justifications for killing those who disagree with you. That's why I find this a better book than Jingo, which is also anti-war--but with a more "patriotic" theme. That one is the book where he satirizes nationalism (Jingoism is defined in the Oxford English Dictionary as "extreme patriotism in the form of aggressive foreign policy". )
Sam is the target of The Summoning Dark because he has a well of anger at stupidity and cruelty. But he is able to defeat The Summoning Dark, because he knows the darkness of his own soul and chooses to control it--by creating the Guardian Dark. Certainly, there are strong religious and/or moral overtones in this.
Does anybody think that what I describe as the humorous thread really works as "funny"? I think it's the one weakness in an otherwise magnificent satiric novel dealing with not only the problems of our times, but those of human nature.