Sometimes newbies also wonder why their lengthy posts that is on-topic is also deleted by the mods, well they don't put it into consideration that even posting repeated answers makes their post irrelevant to the topic and cannot be considered as helpful for the thread which makes them a target for members reporting it to the mods. Not only reading the OP isn't enough anymore you must also consider what others have been contributing to the thread and must know what you will add new to it next.
In some boards, people simply visit, and write anything they think of, in their perspective, and don't care about what already discussed in above/ previous posts. Somehow their posts are off-topic, and off-context.
Admin mentioned about role of length of post in good posts. It is just one of minor factors and itself is not enough to build up a good post.
oreover, 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.