In my opinion, the hardest thing on learning English is its spelling - many languages have "write as you hear" spelling, or something pretty close to that. Although there are some spelling rules in English, most people just have to read through large texts for their brain to accomodate the way English words are written. As I see it, there are even exceptions in exceptions ;-)
On the other hand, conjugations and declensions are simple to nonexistent in English, which gives students time to absorb the rest of grammar (mainly pronunciation, tenses, and structure of sentence).