'This is probably the first democratically killed dragon, one man, one stab.'
If I were to pull out one theme from this book I would have to say that it is about the fickleness of the mob. Basically the mob, or as in this book, the population of Ankh-Morpork, simply switch and change throughout but really have no understanding about what they really want. It is not about the overthrow of an evil tyrant to be replaced with a good king, particularly since in Discworld there is no such thing as a good king. Nor is it about freedom through democracy, because in reality democracy is little more than mob rule. As Zizek says in Living in the End Times the problem with democracy is that the government is focused only on pleasing the mob so that they might get themselves re-elected. In fact, it is the totalitarian state that must control the mob due to their illegitimacy as elected governments (as Howard would always do during the time he was prime minister of Australia) would always lean on the fact that they had the mandate of the people.
As Pratchett says at the beginning of this book, Guards Guards is dedicated to the characters who in most fantasy novels burst into the room only to be systematically killed. These characters are four members of the Night Watch: Captain Vimes, a perenial alcoholic who knows that his job is meaningless because the Patrician has established Ankh-Morpork to keep its own law without the need to resort to a city guard (namely by handing the power to regulate crime to the various guilds – effectively privatising law enforcement); Sergeant Colon, who is a family man that actually seems to be the guy who runs the night watch; Corporal Nobby, who is basically not the brains of the outfit; and Carrot, an Arnold Swartzenegger sized guy who believes he is a dwarf because he was raised by a dwarf, and has a innocence about him that makes him believe that the idea of the Night Watch is all about upholding the law.
This story is unique in many ways because despite being set in a fantasy world, it actually plays out like your average cop movie. A secret society is trying to overthrow the government and does so by stealing a book on summoning dragons, and then summons a dragon to terrorise the people. When the people have been sufficiently terrorised, they bring in a hero to kill the dragon and set him up as king. However there are a couple of problems, namely that dragons, or at least the noble dragons of fantasy literature (as opposed to the swamp dragons that appear in this book, and are so fragile that they could explode at any moment) don't exist, and even if they did, the laws of the Discworld universe make them impossible. It is interesting that despite being a fantasy world, Pratchett seems create all of these impossibilities, such as the idea that a wizard should not rule because if a wizard were to rule then it is quite likely that the world would come to an end (Sourcery), and the idea that if the gods actually have existed then there would be chaos (Pyramids).
The rather exotic and unique characters that Pratchett creates also sets his books apart from your standard fantasy novels. While we have always known about the librarian, and while he still remains a minor character, he does play a larger role in this book (he becomes deputised into the watch). Obviously we know that we don't call him a monkey because bad things happen if you do. Then there is Errol the swamp dragon who simply seems to eat everything. Mind you, Errol does actually have a major role to play in the book, but the timing of Errol's entrance, as well as his mistress Lady Ramkin, are perfect. I notice that despite them being important elements in the story, we are not introduced to them until at least a quarter of the way through the book.
The idea of ruling a society is also interesting because Discworld is not a universe in which the duality of good and evil exists, but rather differing shades of evil. The Patrician is not necessarily a nice person, nor would you consider him a benevolent monarch, but it is clear that to replace him with another king, or even with mob rule, would be even worse. It is not that he is a good monarch, but he is a monarch that provides stability. I think the entire premise of the book is summed up clearly in this passage near the end:
'Down there,' he said, 'are people who will follow any dragon, worship any god, ignore any iniquity. All out of a kind of humdrum, everyday badness. Not the really high, creative loathesomeness of the great sinners, but a sort of mass-produced darkness of the soul. Sin, you might say, without a trace of originality. They accept evil not because they say yes, but because they say no.
'Oh, yes. We're the only ones who know how to make things work. You see, the only thing the good people are good at is overthrowing the bad people. And you're good at that. I'll grant you. But the trouble is that it's the only thing you are good at. One day it's the ringing of the bells and the casting down of the evil tyrant, and the next it's everybody sitting around complaining that ever since the tyrant was overthrown no-one's been taking out the trash. Because the bad people know how to plan. It's part of the specification you might say. Every bad person has a plan to rule the world. The good people don't seem to have the knack.'