I was pretty disappointed in Raising Steam, not because it was bad per se, but because I always look forward to the next book by Pratchett. Perhaps my standards are too high?
I found the characters way too bombastic. They spent so much time winking mischievously at each other and talking about what scoundrels they were, that they didn't do much that was mischievous or scandalous! Though, to be fair, King does like to talk about himself. Maybe the difference in this book is that he plays such a prominent part that he just kind of started wearing on my nerves.
Another problem I had was with the pace. Others have compared the book to an actual steam engine; starting slow and gathering speed. This is why I drive an internal combustion engine. In a book predominately about a steam engine, you had to read a third of the book before the engine even moved! And it wasn't until you were halfway done before it did anything more than go around in a circle! The three big action scenes were also too brief and lacking in detail to deserve the build-up that they got, especially when Rhys returned to reclaim her throne. The fight on the train wasn't too bad, but it was the same thing over and over; dwarf jumps onto train roof, Moist knocks dwarf off or out, and Vimes gets them all to spill their guts with a friendly little chat.
One of my favourite things about the Discworld books are spotting the cultural references inserted into the stories, followed closely by the clever puns. For example; I found Monstrous Regiment more interesting after reading accounts of women dressing up as men to fight in the American Civil War. Some of the stories I found online appear to have directly inspired several of the characters, though that could easily be a coincidence. In recent books it seems that what we get instead are funny accents that can sometimes be difficult to decipher.
I was also disappointed with Moist. First of all, after resurrecting the Post Office, getting the city started on paper currency (which improves the economy and allows Vetenari to fund his projects), and saving the city from the ancient golums, it seems pretty unfair for Vetenari to once again threaten to execute Moist if he doesn't do something impossible. Add that to the fact that Moist ought to have considerable leverage over Vetenari since he knows the secret of how to control the giant golum army and it seems out of character for him to meekly accept his fate and even volunteer his life on a very risky guarantee. Moist has always liked a little danger in life, but he was always sensible and never suicidal.
Lastly, I've always thought that Pratchett had a habit of recycling favourite jokes in the Disc books, but I'm getting a little tired of seeing Vimes. Don't get me wrong; he's a great character, but he's been in Men at Arms, Feet of Clay, Jingo, The Fifth Elephant, The Truth, The Last Hero, Night Watch, Monstrous Regiment, Going Postal, Thud!, Making Money, Unseen Academicals, Snuff, and Raising Steam. And, of course, "Where's My Cow?". That's more than a third of the Discworld books! Actually, what I'd really like to see, is a book that features Esk (from Equal Rites) as an adult. I'm curious about how she turned out.