[Tharkun]

The biggest difference between natives and non-natives is the amount of "passive vocabulary". Sure, we non-native English speakers can express ourselves well enough, we can participate in discussions, even on a high scientific level, as long as we're familiar with the problem domain. But because we've (I'm generalising, of course) read fewer English books, been exposed to the lingo for a lot less than natives, we're limited in our passive vocabulary. Having been exposed to a language for every waking hour of your lifetime doesn't just enable you to speak it better, it also teaches thousands of words you never use, but nevertheless know the meaning of.

On a side note: this makes me feel better about my 25k score ;-)

[eru]

For me it's the opposite. My passive vocabulary is rather big, but I can't think of all those words when I am speaking.