It's probably worth noting that most suggestions do not result in listings. No luck involved -- the editor simply didn't find any significant unique information.
Which is either either bad website design (the navigation didn't clearly lead to content) or spam (the site worked as designed, but didn't offer anything a surfer couldn't get elsewhere.)
There are really two hurdles to pass. A site has to be about a subject that seems under-represented on the net, so someone would be interested in improving that topic in the directory. If it doesn't pass this test, then who (apart from its owner) will care whether it lives or dies? All the world's surfers have a lifetime of online reading without it!
And within that topic, the site has to stand out in some way from other sites. "Stand out" doesn't mean loud self-praise or garish colors. It means, simply, that a visitor can quickly see who built the site, what their qualifications are, and what information they provide. "What kind of information is on this site? How reliable is it?" A good answer to that question WILL stand out.