This thread is getting me very excited! The parallels between first- and second-language learning and other areas of learning are something I've been thinking about for decades. It's at the crux of the Suzuki approach to music learning. Fillyjonk, you've hit the nail no the head here, I think. We tend to underestimate and trivialize the learning process that children go through. When they whine and complain about things being hard, we assume these are childish complaints. When they behave badly or have emotional meltdowns we assume they're just reacting in immature ways to things that adults can handle. But sheesh, I think being a kid is really hard, in large part because you are expected to learn stuff that *big* and completely new to you.
Years ago I read a debunking of the myth that children in language-immersion schools learn a second language so much more easily than adults. The writer asked the reader to consider exactly what the process is like for those kids: six hours a day five days a week of language immersion for several years with plenty of repetition of a somewhat controlled progressive vocabulary, reinforcement through daily activities and the social support of peers doing exactly the same thing. If you could get adults to put themselves in a similar environment for a similar length of time, they'd almost certainly emerge with at least the same vocabulary and facility with the language -- but they'd evaluate their skill level as lower because their accent might not be perfect and because their expectations for a rich, abstract adult-type vocabulary would be higher. If we appreciate how hard children work to learn, we'd have a lot more respect for their learning -- and we'd be less likely to use the excuse of a magic critical period having passed when it comes to our own learning.
I could go on for hours about parallels and differences between language, math and music learning, critical periods, etc. etc.. Unfortunately I'm at work right now, and tomorrow I'm on a plane and may not have much internet access for a while. Hope to find moments to think and write a bit more. Don't have too much fun if I'm not around over the next little while!