Kathy Hutchins is a little more down on the Wallace and Gromit movie than I am, but I too was disappointed. I could have done without the scattering of vulgar jokes, but the whole thing simply wasn't nearly as clever as the W&G shorts.
...you do realize if you're seeing a Wallace and Gromit film for the plot, you're a sad human being.
But it may be precisely the problem of plot that keeps this movie from living up to the shorts. And by "problem of plot," I mean they had to have one that would last an hour and a half. They wound up with one whose conventions outweigh its quirks.
Which is not to say it's not an entertaining movie; it has a 95% approval rating at Rotten Tomatoes, suggesting it's a nice palate cleanser for the critics. It's just not top quality Wallace and Gromit.