For me, it wasn't so much a matter of style, but of thinking that Vimes didn't really have a character arc here. He's already been through the exact same testing he went through here -- the risk of stepping over the line into evil, and not giving his family enough priority -- and came through with flying colors, so there wasn't any real reason why he'd lose this time. I didn't feel the stakes. I love Vimes, but I'm not sure he was the best protagonist for this particular story. I'd like to see a new character introduced -- the way Moist was new in Going Postal -- rather than seeing Vimes go through a challenge he's already met.
OTOH, I so hope that Pratchett is going to write more about Vetinari and the puzzle lady. The one thing on Discworld that Vetinari can't control -- the puzzle lady -- is bound to give him a real character arc. I can't wait for their battle to begin (if, indeed, it's in the works; this is just speculation on my part)!