I think the pitch for this movie went something like this: Remember Krull and Beastmaster? Wouldn't it be funny to make a movie like that, except have Danny McBride star as the lead character? All the other characters would do a British accent except for McBride. And he could act like every other character he ever plays, meaning he swears a lot and is always horny. Oh, and we should throw in a lot of stoner humor, cause that's funny. And we could also have the characters drop the f-bomb a lot. That's funny too.
The biggest problem with this movie is it's a spoof of movies like Beastmaster, but they filmmakers don't know how to do that correctly. In a good spoof, you first have to have a solid story with good characters to root for. For more on that subject, please to be reading this explanation by David Zucker on why Airplane! worked better than Top Secret! They had a decent premise to work with.
Prince Fabious (James Franco) is the dashing hero, and his brother, Prince Thadeous (Danny McBride) is his disappointing brother. There is an evil wizard named Leezer (Justin Theroux) who has kidnapped Fabious's fiance Belladonna (Zooey Deschanel), and the princes go on a quest to rescue her.
The biggest problem is the characters are not written well. We hate Thadeous because he is a selfish dick, and his transformation into a hero is not believable. Also, the elements of the quest are not interesting. They just go from one set piece to another. One sequence has them captured by beautiful topless barbarian women, then thrown into a thunderdome where they have to fight a warrior, then a monster. Another sequence they are in a labyrinth where they have to fight a minotaur (of course).
The filmmakers think that pot references are hilarious. Once in a while they made me chuckle, but no more than that. You actually have to do something interesting besides just referencing pot to be funny. And having a character randomly add the word motherf***er to a statement is not funny.
An hour into the movie, Natalie Portman shows up. She is the most competent warrior of the bunch, yet she inexplicably falls for Thadeous. That is the least believable element in the entire movie. The biggest problem with the movie is it isn't funny enough. I checked my watch every 10 minutes.