Voicing heresy here -- picking up stuff about Pratchett for many years second-hand, I had the strong feeling that his books wouldn't be for me: too much of an impression got, that for my tastes, his humour would feel heavy-handed and leaden and too suffused with an authorial self-satisified attitude of "aren't I just the funniest and most original writer ever?".
As much as I really enjoy Pratchett, I think I see where you're coming from, at least in regards to the early books. They are, for the most part, "gag" books, and I personally believe it to be a result of Pratchett being a tad too influenced by Douglas Adams. Except that Adams was being much better than Pratchett (or just about any writer in the world) at simply playing with language for language's sake, and make it fun for the reader as much as for himself.
I think that Pratchett's books have for the most part improved as he's given more time to plot and character depth (things that Adams was rather less adept at compared to being funny). And some of the things I like the most these days are for example the parts in "Carpe Jugulum" where Granny Weatherwax is alone, and those parts actually doesn't contain a single gag or pun. Or the fact that the first Tiffany book is dealing with death in a far more serious manner than the rest of the Discworld books (interesting really, considering the Tiffany series are supposed to be for younger readers).
This change has of course been gradual, but I would say that around books 12-14 or so (like "Small Gods") are where you can see that he's starting to find the balance between gag/plot/characters.
Somewhere earlier in this thread, someone mentioned that s/he considered some of the books "dark books", which might be more to your liking. Perhaps "Carpe Jugulum" would be a nice place for you to retry reading the series. Or "Night Watch". Or maybe not. The style has most definitely turned more balanced, but it's still fairly recognisable as Pratchett style. So take my post with as many pinches of salt that you feel necessary.
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Oh, was that too serious and polite a response? Well then, unless you promise to worship Pratchett in all his glory, I shall give Nanny Ogg - undisputed matiarch of the Ogg clan - a hint that you might have insulted one of her relatives. -THEN- you'll definitely be sorry, you heathen!!!!