I've dealt with this issue on multiple versions of WP, not just this latest one. WP insists on rewriting code, changing <div> tags to <p> tags. The <hr> tag seems to be completely unusable.
I recently fixed the <div> re-write using the information in this post: http://wordpress.org/support/topic/102799?replies=18
...but it seems like this kind of fix should be unnecessary.
I realize that many WP users don't want to deal with code. But CHANGING code makes WP an overly protective nanny.
I create web sites for a living. More and more frequently, clients are asking for web sites that they can edit. There needs to be some way that I can add in code such as <div class="HomeBox1"> for the formatting power, while still allowing my clients to use the visual editor.