I'm completely biased because Sir Terry is my favorite living author, but the books range from good to absolutely amazing. Yes, they are charming and light-hearted for the most part, but Pratchett has this way with suckerpunching you with some heavy stuff and some of his stuff is utterly heartbreaking, more so because you expect it to be just a funny little tale, but that's not always the case.
While Terry is pretty good at keeping a good plot going, the characters are definitely the standouts. They change and grow in very organic ways, and so does the world which I have to count as a character itself. This isn't like your other fantasy novels where the world is stagnant.
As for reading order,
this was the most recently updated I could find. It looks a bit complicated, but just think of Discworld as a common setting for many smaller, mini-series. Where you want to start depends on you.
The Wizard Series starts with
The Colour of Magic and consists of his early work. It's funny and entertaining, but nothing compared to what comes after. I don't suggest you start with these first though because Pratchett is still trying to figure out what he wants to do and you can tell his first focus are the jokes and not the plot.
The Death Series starts with
Mort which is what I usually recommend as a starting point. It's short but captures a good glimpse of what's going to become Terry's more mature style, but is still early enough in the series that it does a very good job of introducing how the world works. I'm trying to distill the the Death Series into a certain genre, but I can't. It jumps all over the place, from one novel being a love letter to rock music, another about retirement, a Christmas novel about the power of belief in stories, and time travel story. The stories revolve around Death and his family.
Another book I recommend to new readers is
Small Gods. It's one of his best and chronologically it takes place before all the others. It's probably the first time Pratchett figures out his voice and manages to hit that sweet spot of being utterly hilarious, insightful, heartbreaking, and heartwarming all at the same time. Although some friends have thought it to be too critical of religion, so you might find that as well, but in my opinion Pratchett writes his messiah figure with a certain warmth and not bitterness.
The Witch Series technically starts with
Equal Rites but the novel is kind of ignored in the continuited, so I would start with
Wyrd Sisters. It also features one of Pratchett's strongest protagonists, Granny Weatherwax. The Witch Series primarily deals with fairytales and Shakespeare, so if you enjoy either you'll have a lot of fun with these. They turn into Young Adult novels with the start of
The Wee Free Men, but don't let that stop you from continuing to pick them up because I've found that they are just as funny and engaging (and it must be said, somehow manage to be sadder, in my opinion) as the adult ones.
The Watch Series starts with Guards! Guards! and follows the police force of the biggest city on Discworld. So mystery, politics, that kind of stuff. It also stars one of my favorite characters in fiction, Samuel Vimes, and he has probably the most robust character arc in the series.
There's also the "Industrial Revolution" novels which have similar themes (introducing new concepts to the world such as the printing press and the post office) but different characters. You should read these in order if you want to see how the world progresses and catch cameos, but otherwise it's not necessary.
