As promised, I'm following up Guards! Guards! with Men At Arms. This book revisits the Watch and marks the transition from single book to a series about the guards. There's an appropriate amount of continuity (and joke re-hashing) as new characters are added. It's very enjoyable, but not quite as successful as I remember.
I think the fundamental issue is that it tries to cram too much in. It deals with racism, monarchism and guns all in the same book. I think the "gonne" is the weakest part. It follows in the path of other "Earth thing turns up in the Discworld and wreaks havoc" books, like Moving Pictures and Soul Music, but just doesn't really pull it off. Meh.
The racism angle is tricky, and perhaps nowadays there'd be hand-wringing about it being written by a white, male author, and how it's Problematic. As it is, handling racism and long-running racial feuds works pretty well in a universe with fantasy stereotypes and not so much different races as species. Combined with the fact that most of the citizens of Ankh-Morpork are low-grade awful, and a few are trying to improve themselves, and you get something that at least addresses "racism is bad" without anything being painfully out-of-place for the Discworld. That's this white male's simplistic view, anyway.
If anything, the awkward bit is Angua and Carrot's relationship, where she seems to be pretty much his pet. Mind you, looking at Vimes and Sybil, I guess Discworld relationships don't need to make that much sense.
Anyway, the monarchy theme from Guards! Guards! is back again, and... it's just laid on a little thick. An awful lot is written about Carrot's likeability, perhaps because it wouldn't be clear otherwise, given how stolidly boring he is (not that he's written boringly, he's deliberately written to be boring).
So, all this makes it sound like I'm really down on this book. I'm not. It's still very entertaining, and the evolution of the Night Watch into the start of an actual police force is fun to watch. Perhaps the thing is that despite the overall themes being handled in a lacklustre manner, the page-to-page writing is still excellent. There are some great set pieces - in particular, I love the pork futures warehouse. And I'd forgotten that actual foreground characters you care about get killed (spoiler alert!). Let your eyes glaze over a little on the big picture and enjoy the ride.