. . . it's not.
Mind you, I prefer my Slayers of the Buffy variety and Jack is trying a little too hard in places, but no, it's not awful. The story hangs together (even if it's feels a little long), you have some nice effects, and the actors are clearly having a very fine time. So why isn't it a smashing success?
Think of it this way. I have a sweet tooth - I'm pretty much a candyfreak. While others want to discover how a creative barman can mix pomegranate juice with various alcohols, I'm way more curious about what it's like to dip the somewhat-exotic seeds in dark chocolate. I'm a card-carrying member of the Lindt chocolate club, but I can also be quite happy in the dollar store by the movie candy. But you can have too much of a good thing - European truffles and Sugar Babies and red hots and Now 'n' Laters and caramel creams - well, that's just Not Good.
That's the problem with Jack. I don't know if Bryan Singer (who directed the flat-out brilliant Usual Suspects, as well as three X-Men movies and the upcoming Days of Future Past installment of that franchise) had Suits poking their noses into his production or what, but there's just too much. It's like me in a candy store with low blood sugar and a credit card - everything seems like a good idea! Not just one giant - a whole country of them! Not just one beanstalk - how about seven or so? A farmboy and a princess who share the same taste in bedtime stories! Magic beans! Monks! (Monks?) Don't get those beans wet - wait, are we in Gremlins all of a sudden? Political intrigue! A magic crown!
Nicholas Hoult (who was Beast in Singer's X-Men: First Class and with his turn in Warm Bodies has become one of my Young Ones to Watch) is Jack and this "Farm Boy" is very much cut in the form of Westley from Princess Bride. Well, if you're going to crib, pick the good ones, I say. There's more than one touch of Princess Bride in Jack, including the use of a voice-over story as a framing device. However, these giants are not nearly as gentle as Fezzik. The giants are gross, slobbering monsters who have a low cunning, but not much in the way of actual intelligence. I daresay kids would be entranced by the fun the movie has with scale - full-grown men come up to about knee-level on these giants and the beanstalk is fantastic - and yes, beanstalks work both ways as chlorophyll staircases.
Look, Ewan McGregor, Stanley Tucci, and Ian McShane (now with 75% less cussing than in Deadwood!) are all having so much fun chewing the scenery that they need to floss plywood from their molars. Not much for the women-folk to do here - and being a giant seems to be a male-only sort of thing. Princess Isabelle wants to have adventures, but is basically a prop to be rescued and won. (By the way - golden armor? Really? Face it, princess, it looks pretty, but won't stop a butter knife.) Then again, not every fairy tale can be Brave.