In the new dialog, you can set the title, description, link to an image or decide if you want Disqus comments enabled for this post only.
The first three are included in the Twitter and Facebook metadata for the post, so if you want to override the title, or provide a description that isn't the full text of the post, you can do that in this space.
Further, you can set the image that is displayed in Twitter or Facebook with the post. That's how I've been getting my posts to have cool images for each post. It's a little extra work, but it looks so much better. Here's an example of a Twitter post and the same one in Facebook.
Here's a view source of this post to see how the metadata is represented in the JSON for the post.