In Guards! Guards! we meet the Watch, comprising very few members at this point. Each book adds more and more members, which is one reason it's nice to read them in order. Another is that various characters move up the ranks over the course of the books. And another is, of course, Samuel Vimes, many people's favorite character in all of Discworld. It's hard to believe when we're first introduced to him in a drunken stupor, but he cleans himself up, dammit, and proves himself to be a great copper. The current Vimes has come a long way from the man lying in a gutter. In any case, Our Heroes have to protect the city from a dragon, but it gets...complicated.
Men at Arms, as its name suggests, focuses on a weapon: there's an assassin on the loose! There are murders to be solved! One reason I really like the Watch books is that they are by their very nature mysteries, and this book in particularly features some clever detective work, both by Vimes and one of my favorite characters, Carrot, a human raised by dwarfs whose sincerity is only matched by your disbelief that he's sincere.
Feet of Clay is probably my least favorite of the seven. The Watch has to deal with a potentially murderous golem. I don't remember quite why I wasn't as fond of this book now, but I wasn't as drawn into the mystery and the golems, I guess, even though I still enjoyed the stuff going on with the Watch characters.
Jingo is the longest of the seven, and it really opens up the story and begins to introduce more politics into the mix, as Vimes has to stop a potential war between Ankh-Morpork and the Klatchian city of Al-Khali. This book is the one that ensured Vetinari a spot in my list of favorite characters as well. The most brilliant thing about what Vetinari does in this book is that it is completely between the lines and never stated outright.
The Fifth Elephant continues the theme of politics as Vimes travels to Uberwald and faces werewolves and vampires and dwarf kings. And of course he has to solve a mystery before there's an international incident. This is one of the more complicated plots, but it also has a strong focus on character and character relationships.
Night Watch is unusual in that it does not feature most of the Watch besides Vimes. Well, at least not the version we know...because Vimes is transported thirty years back in time. Yes, my friends, this book features time travel mayhem. (I didn't lurve it as much as I would have expected, given that, however, I think because I missed most of the characters I had been reading about, although it was neat to see younger versions of some of them. One especially.) Vimes has the misfortune of landing during a very important but violent time in Ankh-Morpork's history, which he has to become a part of if he's going to make it back home.
Thud! focuses on the contentious relationship between trolls and dwarves that goes all the way back to the Battle of Koom Valley, when the trolls ambushed the dwarves. Or the dwarves ambushed the trolls. No one is certain of the details, but everyone knows that doesn't stop hate and prejudice! Once again, Vimes must solve a politically charged crime to keep Ankh-Morpork from exploding in violence.
I've already touched on some of the things that make the Watch books stand out, like the mystery aspect and the character development. Pretty much all the major Watch characters are likable and entertaining, though I never really warmed up to Colon (I've taken care not to name Watchmen past the first book, as part of the fun and surprise lies in the identities of new recruits: there are a lot of non-human members, I'll tell you what). The relationship between Vimes and Vetinari is a thing of beauty, as they're both shrewd in their own ways. They need and respect each other, but they also wouldn't mind if the other ended up in a ditch somewhere. I also liked some of the recurring minor characters like Leonard of Quirm, the kooky inventor.
But I think the biggest reason these books are so lauded and recommended, specifically, is that Ankh-Morpork is a character. Through the Watch, we get a real taste of what the city is like. We meet its inhabitants, individually and collectively. Through Vimes and Vetinari, we see how the city is run. They both love the city in their own way and serve it to the best of their abilities. Even when the characters travel to other cities and countries, Ankh-Morpork has such a reputation that we get a picture of how it fits into the larger world.
So if you're a Discworld fan but haven't read the Watch books yet, I do endorse reading them in order! If you're not a Discworld fan, what the hell is wrong with you, there are like thirty books, just pick one up.