1. Lame plot.

The ordinary teenager from the real world who gets transported to a fantasy kingdom (which he then must save before returning home). Any narrative that employs this lame plot is digging itself a huge hole right at the start, and will have to be exceedingly brilliant to make up for it. There are lots of good reasons to avoid this plot, and especially the implementation in Forbidden Kingdom:

         2. Lazy writing.

This plot is a way to avoid a couple of the challenges of writing in a heroic fantasy setting: the need to explain the special rules of the fantasy world to the audience, and the lack of a character the audience can relate to in a cast populated by legendary heroes. Dragging in a character from the real world is an easy solution to these problems that saves the writer from having to do anything sophisticated (such as a show-don’t-tell approach to presenting the setting, or writing complex heroic characters with realistic flaws and motivations).

       3. Insulting the audience. 

It’s really not that hard to stick a relatable character in a heroic setting—even young children recognize that the farmboy or the hobbit is supposed to be the audience stand-in. The “ordinary teenager from our world” is the most literal interpretation of the relatable character (short of a Choose Your Own Adventure story told in the second person) and suggests that the writers didn’t think the audience could handle anything more subtle.

      4.It strains suspension of disbelief.

A self-contained fantasy world is easier to accept than one in which people from the modern world are randomly popping in and out. Furthermore, it wrecks the sense of otherworldliness to have someone walking around wearing jeans and spouting American slang. There’s less immersion with a constant reminder of the real world in the center of the frame; a good fantasy should make the audience temporarily forget where they are.

      5. It contains disturbing racial overtones. 

This plot device would have been bad enough if the teenager had been from Shanghai. But the use of the Boston setting and Michael Angarano suggests that the filmmakers decided that, in a film set in Fantasy China, a Chinese lead wouldn’t have been white enough or American enough for American audiences. Now, it may be true that the film will make more money with an American lead (although it’s interesting that he doesn’t appear on the posters). Nevertheless, such shameless pandering is ugly. And equally disturbing is the imperialist notion that powerful warriors like Jackie Chan and Jet Li couldn’t save China until the American (with no special skills or talents) showed up.

Mostly via  http://moviehatersloveu.blogspot.com