I've had several inquiries from people who are confused by this change, so I'm making an announcement about it.
When you reply to a message, the default reply subject (starting with "Re: ") should be in gray as placeholder text. If you don't have a modern browser, the default subject may be blank.
If you start typing in that field, the placeholder text will disappear without you having to delete it.
Before, in order to clear the default text, you needed to either select it and delete it, or click the little X on the right of the subject field. This makes that unnecessary. If you wish to use the default subject, just leave the subject line blank (with the default text as the gray placeholder text). The board will automatically use that default subject line when you post your message.