The new Comment field - When filled in, the contents of this field displays immediately after your name on the Index page in Thread view (it currently doesn't show up in Subject view or on the Show page).  It's mostly cosmetic, but does have a useful purpose.  It allows you to edit that comment independently of the name, and it makes editing posts later easier whether you have an account or not.  It will eventually lead to a point where changing your account Display Name (if you have one) will automatically change the display name on all your posts (without the comment field that's impossible).  This field is Rich Text compatible.

The Alternate Login - If you're not logged in, you can use this field to temporarily log in just for one post (but not permanently - when the post is submitted, you'll still be logged out).  Or you can post with a different account.  If you're posting from a shared computer this is an ideal way to post without staying logged in at a strange location.  This feature will give you quite obvious feedback as it checks the account name/password.  Obviously Javascript is required for this feature.

Layout Changes - The posting form has been neatly centered and has a new crisp, clean look.  The attachments links have been moved to the right to keep the buttons close to the Message box instead of an entire page further down.