I updated OP with the point #7, from theymos. Admin clearly emphasize that post quality does not soley depends on your post-length and your English. They are componental factors, but minor ones. Ideas are more important, and writers have to put decent efforts to read, learn, have their own collections of good posts, threads, categorize them for later use. If you see a good posts, threads, but you don't note them down, next time you will have to search and it takes more time to find them (sometimes you don't remember exactly keywords should be used to search)
(7) Ideas that I took from theymos' Writing a welcome messageIf you make ten thousand posts in a week, your activity will be capped and you will still be a Newbie. If you make ten thousand useless posts over any period of time, you will gain zero merit and you will still be a Newbie. You can rank up only by making good posts consistently. It's quality over quantity.
When trying to write quality posts, a lot of people act as though they're writing a book report for school: putting facts that we already know into their own words. Nobody wants to read that, and you will not get merit for it. Moreover, the length of your post and the quality of your English are only minor factors. In trying to write a quality post worthy of merit, you should offer new ideas, personal experiences, or perspectives that other forum users will actually find new and interesting.