1) Contents: It is very important. You should compose threads which are original, innovative to catch attention of other users. You should not copy and paste; or more seriously plagiarise because those behaviours violate the forum rules.
Check it there.
2) English grammar and vocabularies: They should be used as correct and appropriate as possible to help readers understand what you write and want to describle through your threads.
3) Contexts: Your threads should answer questions in context. Most of users jump directly to answer the question of topics' OP. They don't care about whether it was answered and solved by other users in previous threads after the OP or not. Posting, answering like this is likely a spamming thread.
4) Formats: Don't overuse bolding, highlighting, etc. because those effects have its own side-effects which makes readers feel uncomfortable, make threads harder to read at some extents.