For me, a quality post is informative, often entertaining, in-depth and answers the question posed while adding new questions to ponder. The typical 'good post' will be at least a paragraph long, not just a 'thank you' or 'good post' type comment. Most importantly, the poster's personality and voice comes across clearly in the post.
I'm not so concerned with length of a post (provided it's not like 3-4 words) since not every post needs to be a paragraph or more. But enough to show real thought has gone into a post? Most assuredly.
I also expect links to be kept relevant to topic. And not to advertise their own works. (So no cases where a user goes "I talked more about this subject at my site".)
Quality for me, usually means that the person has atleast tried to type out 5-7 words, used good spelling, grammar, and tried to not look like it is just to up their post count by 1.
Good spelling and grammar I think is something that's a must. There are members where English is not their first language, so in that case I don't think they need perfect grammar. As long as the post adds to the conversation, I consider it "quality."
A 1-2 liner is ideal. You want to express your thoughts properly, so use descriptive words to add more emphasis. Random words and nonsense will not count as a quality post. Add a question or two, and your very own opinion on the matter at hand. 😛
