Meriting is subjective, and not subject to a specific criteria. A common denominator would entail the posts being of interest, helping out, stating a well versed opinion, and so on. Style is diverse, but generally it should envolves the content to be easy to read (i.e. not written in broken-English – there are local boards for those not well versed in English).

Length should not be a factor (providing it’s not a one-liner garbage post), but placement does have a weight in visibility (i.e. Altcoin boards may be more difficult that say Beginners & Help, and posts on tails of mega-threads are generally not that good at obtaining merits).

Some posts obviously may go unmerited for multiple reasons, but on average, if the poster created decent content, he will be merited on some of his posts. Even the best posters may have averages of being merited say in 1 out of 5 posts (although the merit per post ratio may be higher if the amount of merits received on the merited posts is high).

Normally it all boils down to the content provided, the expressivity used, and the time one has to create that content (correlated to the amount of thought put into the content).

Note: Looking over your posting history, you tend to overly quote other people’s posts entirely. It is better to delimit the quote to the part you want to respond to, and not the whole post, in order to render a better understanding and read.

Note 2: If you want to see it from a quantitative perspective, you can play around with the Merit Dashboard that figures on my profile.