What makes English so difficult to learn?

Why many foreign speakers have such problems with the English language? I mean, I came to the U.S. about two years ago, and in the beginning I've had problems with the language (probably just like eveybody else), but what makes English such a difficult language to learn? Most people (like in Europe) they grasp the [other] languages right away. The pronunciation is a lot easier, but still? What is so difficult about English? I know that there is stress, and that a lot of people shorten the vowels, but what makes English so hard?

Thank You for taking Your time reading this