The Long Earth is nothing like the Discworld books: it's arguably fantasy rather than sf, but it's definitely not humor. It's mostly about exploration and social/political/cultural change in the face of new technology. The premise is that mankind discovers how to travel to an apparently infinite series of parallel Earths by "stepping", one Earth at a time to the next Earth over. There are two directions of travel, arbitarily labelled "east" and "west". All of the parallel Earths (at least at first) appear to be devoid of human or other civilization: they're untamed wildernesses.
A lot of the book is about how humanity reacts to this development, which gives the reader plenty of time to consider the authors' various assumptions about human behavior and desires. In my case, this was way too much time to think about it. I could suspend disbelief and accept the parallel Earths without difficulty, but I frequently found it hard to accept the authors' portrayal of its impact. Various details, large and small, would throw me out of the narrative, over and over again.
Some things I liked about the book: the nuns who raised one of the protagonists were quirky, entertaining, and believable, though seen almost entirely through the lens of the protagonist's memories rather than directly. There were rival interests at work in the book, but few villains. The heroism displayed by some of the characters was understated and delightful for that.
But around page 200 or so, I was mostly bored by the book. I didn't feel engaged by the characters and the central mysteries didn't seem to be getting anywhere. I considered dropping it and reading something else. I forgot to bring it with me to read at lunch. Still, I persevered and it did get better towards the end, but I'm ambivalent about reading the sequel. The sequel is The Long War, and the general lack of war in the The Long Earth was one of the things I liked about it. Lut read both and wants to read The Long Mars, but he admits that if this had been the first Pratchett novel he'd read, it might've been the last.
I'm not disappointed that the book isn't humor: I didn't expect it to be, and I tend to like Pratchett books better when he's not trying so hard to be funny. I just didn't find this particular book that engaging. I might give the sequel a shot anyhow -- like I said, it got better towards the end -- but this one comes in at 6, and I'm planning to read something different next.