Headlines are by far the most important because "if it don't get opened, it don't get read!". And of course, the author's bio box is critical because you want that click.
Something that has been largely overlooked, and is critical to the success of any article post, squidoo lens, or social bookmark is very important. It doesn't matter if you are at ezinearticles or digg, you are going to be asked for two things.
They want a brief article synopsis with a word limit and a group of tags that describe the article. I'm guilty of trying to "digg" an article or blogpost of mine only to get there and think, "Oh Hell, What do I use for that?"
Most of the time I wind up using the lead in paragraph as my 'article description' and then trying to attach tags that come to mind to fill in.
The point is, I've started developing these important parts of the promotion and saving them in a text file, so that when I submit an article or whatever to either an article directory or a social site, I've already got an 'optimized' text to use for this.
Hope that helps somebody