I just read a great article on some of the most difficult and challenging aspects of learning Mandarin Chinese. The article is written in a sort of humorous and chatty tone, and brings up some of the most frustrating things Chinese learners face when they really get down to the nitty gritty details of learning.
Not all foreign languages are equally difficult for any learner. It depends on which language you're coming from. A French person can usually learn Italian faster than an American, and an average American could probably master German a lot faster than an average Japanese, and so on. So part of what I'm contending is that Chinese is hard compared to ... well, compared to almost any other language you might care to tackle. What I mean is that Chinese is not only hard for us (English speakers), but it's also hard in absolute terms. Which means that Chinese is also hard for them, for Chinese people.