Personally, when it comes to defining someone as 'high-quality', it's usually due to their devotion to their account. For example, you have a member who rarely makes any posts on the forums, but a majority of the responses he/she creates are informative and you can easily notice that the member has spent a lot of time on their post.
Spelling, punctuation and grammar can also deem someone high-quality, but not always. It's just like the Grammar Nazi award; not everyone who has obtained that award is high-quality, but people presume they are because of a miniature icon. Anyways, you can be foreign and not have extensive knowledge of the English language, but you may be more devoted to assisting users than those who actually have English as their primary language.
The members who lurk in the midsts and in the shadows, who only respond to threads because they wish to assist another user, or they only create a thread because they want to enhance ForumKorner or benefit the members; that's my definition of 'high-quality'. Also those members who will go out of their way to extensively assist a member through private messages because they want to be helpful, and they don't care about being perceived as 'helpful' on the forums for a higher overall post count/reputation.
Those are my two cents.
