Thank you for this thoughtful comment, I think your writing precisely embodies what nevergetenglish was referring to.
> "It is brutally hard for most people (after the age of puberty, and perhaps especially for males) to learn to attend to sound distinctions that don't exist in the learner's native language."
I'm a native French speaker who learnt English in Scotland when I moved there (been living in mostly English-speaking countries ever since). As far as most people I meet are concerned, I have a very weird accent; the perception ranges from "he's Scottish" to "what the hell?" depending on where I go and who I talk to. In any case, while phonology is clearly very important, I would argue that it is not completely necessary for expressing yourself properly, at least in writing.
> "[...] another huge task for any language learner is acquiring vocabulary, and this is the task on which most language-learning materials are most focused. But often the focus on vocabulary is not very thoughtful."
This is sadly very true, though I honestly don't think there's a silver bullet when it comes to learning vocabulary. One additional source I would add to your "royal road to learning vocabulary" would be exposure to colloquial language, or slang at the extreme, i.e. listening to day-to-day conversations between "commoners". This is how I realized that there is often a "correct" way of saying something (as in purely grammatically correct), and the "natural" way (i.e. as expected by native speakers).
> "Every language has hundreds of tacit grammar rules [...] which reveal a language-learner as a foreigner when the rules are broken"
Continuing from my previous point, I think the opposite sometimes applies too. For instance, back in Scotland, I had to force myself to say "If I was" instead of "If I were", because the mistake has become so systematic that it has almost become a rule.
I think grammar is at least as important as vocabulary, particularly in a language like English where the language is so modular that words can fairly easily be made up, to the extent that they are understandable within a logical context, even if incorrect.
Finally, I would add culture as a possible fourth big task to add to your list. This may just be me, but there seems to be a dimension of language closely tied to the culture associated with it. There are idioms that make complete sense to me, yet I cannot explain why to my (French) family. This is ostensibly related to linguistic relativity[1].
[1] http://en.wikipedia.org/wiki/Linguistic_relativity