I think a post should be well written in terms of grammar and spelling, otherwise people might not know what you mean or worse get annoyed and stop reading. Little mistakes are ok. For me personally, a post shouldn't be too long, unless the subject is absolutely riveting. A little humor also goes a long way. You should have a catchy title and a good first sentence, since these will be seen by steemians looking for a post to read. It will be the first thing to grab their attention.