The text box for tags (when posting a question) and for the edit summary (when editing a post) seem like they should be immediately below the text box for the body of the post rather than after the preview text.
Early in my Stack Exchange experience, I encountered the warning message about an untagged question and I think grouping the text entry portions of the form have made the ability to tag posts more obvious. (I.e., after composing the body of the post, I scanned for the submit button. When looking for a button it is not especially difficult to miss an additional text entry field.)
For editing in particular, I think it would be more convenient to have the summary immediately below the body text box to facilitate partial updating of the edit summary while doing an edit. It may also be more difficult to remember specifically what changes were made after reviewing the post in the preview (at least for me).
I can understand putting the post button after the preview as that tends to force at least a scan of the preview text. (More than once, this forced delay and incentive to scan the preview has saved me embarrassment.) If placing these text boxes after the preview is actually helpful in encouraging more careful
I am asking this on User Experience because it seemed more fitting than going directly to the main Stack Exchange meta site (and I already set up an account to upvote some posts).