Initially, I use to think merit is by chance ( i.e something that will just happen) or lengthy article ( i.e writing too many paragraphs and the rest of them), but I have come to realize that, it is more than that.
I have being discouraged several times because of my numerous posts and replies yet no merit then. Even now, some times I use to be discourage. However, the only way to get merit is by posting quality article ( that is what determines merit) which almost everybody is aware of now.
In summary, don't look at the quantity of post you have make so far but instead try to adjust, I mean improve in your posts, above all be consistent and patient that is the key.
You have said it all, a post that's very lenghty and contains no knowledge or be of any help to someone is of no use. Moreover we don't comment on every posts we come across on the form. Look for threads you know you have adequate knowledge about and dedicate yourself there. Some people comments on every posts they come across on the forum, which is not very good. You already know that without quality posts nothing for you.