I think the complexity of the language in question is a big factor as well. Gramatically simple languages such as English or Italian can be learned more or less perfectly through normal studies, so even perfectonist introverts can master the language with reasonable amounts of effort.
On the other hand, when it comes to a very difficult language such as Polish, the perfectionist introvert who always wants to be 100% right can just forget it. The language is simply too hard, and the only options are to speak badly (at least at first) or not at all (or resorting only to short and simple sentences). You can read grammar books, learn vocabulary etc for 5000 hours, but if you haven't developed a feel for cases, inflections etc, you will only be able to speak at a rate of around 1 word per second or so (or slower), and even if your sentence is 100% gramatically correct, it would probably not be idiomatical, because you haven't learned to think in the language in question, but rather translate something from your native language.
I speak from my own experience learning Polish. I would say I'm the introvert perfectionist type who spends endless hours learning new words and studying grammar. I've been living in Poland for around 16 months, and although I know most things about grammar, theory and a lot of words, I couldn't really participate in a normal conversation. Usually I simply don't understand what people say because I have no time to translate in my head, and when I try to speak myself there is simply no time to think like "plural genitive male of "-r"-ending should be "-rów" etc, so either I think for a long time before I speak, or I speak with disastrous grammar and will most likely not be understood. It's not as if I'm not in contact with spoken Polish or anything. I hear it all the time on the radio, on TV, in the shops, and I have a Polish gf whom I often try to practise with (although she doesn't seem to comprehend the meaning of "speak slowly").
The only non-slavs I've heard of who managed to learn decent Polish were the kind of people who didn't really try hard, who didn't have any great ambitions, and who had this ability to absorb what they hear around themselves. They make a lot of grammar mistakes, but they're still understood. Perfectionism is really a big obstacle towards language learning.
