I honestly think the biggest problem a majority of the roleplayers on this site face is that they can't grasp "Show, don't tell."

Examples I have seen recently;

"Gosh! My character is so smart and clever, even though I've done nothing to suggest that!"

"My character's a reeeeeal edgy dick, lemme just have him sulk in a corner instead of taking a perfect opportunity for him to be an asshole!"