English does have the curse (or blessing, depending on how one looks at it) of being more vague than almost any other language. I'll admit that I didn't learn French with the goal of expressing myself actively in the language - rather, I just wanted to be able to read French literature and watch French movies. It's a passive sort of knowledge.

However, that having been said, most languages have subtleties that make it difficult for foreigners. Russian, for example, uses a large number of parasitic particles that mean absolutely nothing, essentially, but put stress or little emphasis here or there. Knowing how and where to use them and in what context is something that I think only one in every 100 foreign speakers of Russian has gotten, if that. It takes time - a lot of it - and a LOT of exposure to the language.

Beyond that, entire books have been written about the subtle differences in meaning that are conveyed through minor changes in the endings of diminutive forms, minor shifts in word order, and even the dreaded "IK-5" tone, which leads many foreigners to think they are being praised when in fact they are being excoriated. Imagine a sarcastic tone that is said in a way that non-natives don't realize is sarcastic.

I'm curious, however, if you could provide an example of a subtle shift in French that wouldn't be picked up on. It seems to me that French has far fewer "hurdles" to perfecting it than languages farther removed, and that most of the hurdles are from the faux amis, as they say.