I think people have hit the nail right on the head with how to make a post enjoyable for the community:
TOPIC TOPIC TOPIC. Find something interesting to talk about! By that, I mean, finding something that the community could have a discussion around, rather than something that people can just answer yes/no to. (Although, polls can be fun!)
Visuals can be helpful! Aside from pulling attention, visuals can help illustrate what you're talking about. For example, if you're talking about a new skin idea, maybe sketch it out.
No wall of text, pls. Sure, you might have a lot to say on the subject, and that's totally cool! Try breaking your thoughts into paragraphs, and posting them logically. It'll be easier for everyone to follow, and people who check out your post won't be intimidated by a bunch of text on their screen. Bolding important ideas is also a good call!
Try to be positive. You may not be a fan of a recent change to your favorite champion. We still want to hear about it! However, keep in mind that a post like, "THIS CHAMP SUCKS, PLS NERF" is probably not going to get a good response, whereas a post like, "I really don't like the changes made [champion's skill] because I think it really makes laning phase more difficult now. Anyone else noticing this?" will receive a better response. Quite simply, there's a discussion to be had, rather than someone just stating a negative opinion, which can turn some people off to responding.
Good luck posting!