My parents spoke to me in both English and French when I was a kid, and I went to French elementary school. So I think I'm pretty fluent in both languages
I've also picked up a bit of Italian and German from my music studies over the years. Having to analyze opera arias in Italian, for example, kinda forces you to learn a bit of the language.
The most difficult thing about French is knowing when to use what verb tenses. As a nearly native speaker of French I can do this subconsciously when writing and speaking, but if I have to explain verb conjugation to somebody, it's really hard. Especially in regards to which auxiliary verb ("avoir" and "être") to use when conjugating verbs into complex tenses. I don't normally have this problem with English as the auxiliary verbs ("have", "do", and "be") and most modal verbs ("can", "will", "may", and "must") are conjugated in the same fashion for most tenses, especially when used together. And, like CC said, the masculine/feminine thing is hard at times.
Languages like French, Italian, Spanish - the Romance languages - are so lyrical in comparison to the Germanic languages like English, Dutch, and of course German, are not. I wish English wasn't such a harsh language.