[With regard to Huffington Post, it might be a bit difficult to apply the 'relevant opinion' principle in qualitative terms to an average GV post. It seems to me that in some cases it is more of a 'how many opinions should I record' problem.

Probably not all posts need to be long. While representing a pattern of similar and opposing views in any single post is probably part of the GV job/mission, appeal to a portion of the readership may fade when an article becomes too descriptive, although there may be cultural implications in the way people perceive length Vs brevity (and description Vs discussion)