Hosmer has legit power, but he is going to have to hone his swing before he can handle higher levels of competition. Along with a tendency to have some length in his swing, he can get a little too handsy, meaning that his hands get out in front of hip rotation (producing "extension") when they should be turning together. It doesn't look to me like he is trying to extend; he is just a little too eager to get out in front. Perhaps he could adjust his hand placement and bat angle to create a swing path that will make it easier to keep his hands back.
If Hosmer can make the necessary adjustments (an easier task given his overall athleticism), fill out his frame as expected, and display what most scouting reports say is an excellent approach at the plate, the Royals might just have a star on their hands.