ChristianBecker, don't feel intimidated by the length of the discussion. You brought up some good, and pretty much different, points about the book--i.e. points no one else had mentioned so effectively!
It seems to me that in this book, Terry (as he loves to do) reminds us of other writers, styles of novels, and even specific individuals. But none of these shape the book's construction and raison de etre as much, I think, as does his exposure of the cruelty, stupidity, and mindlessness of xenophobia. Xenophobia, traditionally defined as fear of strangers, includes racism, specism and to some degree sexism. It can even be seen as an underlying element in religious persecution. And it is used by all sorts to justify the actions or crimes committed against those who are not a part of one's own small group. Oddly enough, it frequently allows the oppressed to accept their treatment by those in power as just and right when it is clearly morally wrong.
Vimes and Carrot guide the plot, though apparently investigating different "crimes." And although Carrot expresses some of Terry's strongest positive attitudes in his belief in the life and need for protection of the golems, Terry also uses him to delineate his absolute blindness as to the sexism in his upbringing and his current attitudes. (Terry will go on to expand on that theme in Monstrous Regiment.)
Vimes, like his famous ancestor, does not believe in "nobility" as it appears in AM. There is no real difference automatically and irretrievably given to nobility. Sybil is (as she is portrayed) an especial fine woman and not all that concerned with the more ludicrous aspects of nobility (as portrayed by TP). But the common people are not noble or superior, either. Vimes has a job to do as a Watchman--to maintain the peace of the city. And thus he has a responsibility for discovering who is poisoning Vetinari and how because they are disturbing the peace of the society.
It takes Vimes quite a time to figure out how the poisoning is being accomplished (longer than it took Vetinari). Carrot, for all his understanding of golems, seems determinedly blind about this aspect of the crime being committed.
The two threads come together, finally, when Vimes takes the literally re-born Dorfl (leaving Carrot and Detritus asleep or unconscious) to arrest Dragon who has stage-managed the plot against Vetinari. Dorful no longer has or needs words in the head. As he says before his rebirth "WORDS IN THE HEART CANNOT BE TAKEN." And as a self-aware and rational alive being, he is a perfect Watchman.