WMGs for Discworld / Men at Arms. Warning: Potential unmarked spoilers.
Vetinari has something of a Heel-Face Turn in this novel, motivated by Vimes and Carrot.Think about it: the Patrician completely failed to read and manipulate the situation correctly. He tried to push Vimes into solving a series of murders that he was already working on, and that attempt actually drove Vimes into retirement. He had no idea that Cruces was the murderer. And at the end, Carrot's proposal to rebuild the Watch takes him completely by surprise. What we're seeing here is Vetinari realizing that his mental model of Ankh-Morpork and of human nature in general is inaccurate. That there are not "always, and only, the bad people." That it is possible to improve the city while preserving civic order, not merely leave it balanced in its misery. It is only after this novel that Vetinari's manipulative skills really begin to match his towering reputation for having them.
Detritus' large equation has an obvious answerWhen he gets locked in the pork futures warehouse, he creates a huge, elaborate "theory of everything" equation all over the walls, in an attempt to pass the time until he is rescued. He gets everything down to one last "=" before being rescued. I suspect that the answer on the other side of that equals is 42. The Discworld Universe has the Question, and the HHGG universe has the answer. Unfortunately, because this universe has seen the answer, we'll never find out the details of Detritus' question.
Detritus' large equation has an even more obvious answerThe universe is what it is. Deal with it. Alternatively: Nothing's nothing, stop worrying.
The Lord Rust mentioned in this book is not the same one as the Lord Rust in later books.He could have died between Men at Arms and Jingo whereupon the Lord Rust we all know took over.