I set up all of my web design clients on WordPress because it's super easy to use. Most of my work takes advantage of the featured_image functionality to set thumbnails for posts, header images, etc. The problem is, the current UI for setting a featured image is way too complicated for my average client.
When I click the "Set featured image" link, the first page with "select files" is just fine. The problem is, on the next page, the "use as featured image" link is a tiny text link buried among all the other settings. WordPress should automatically set any uploaded image as the featured_image when the user uploads it from this dialog -- ie, only when the get to it from clicking "Set featured image".