Announcements could be more powerful if they supported the following:

1) Expiration: Many announcements are focused on outage maintenance windows which end in the middle of the night. It would be great if these could be hidden automatically.
2) Support for HTML, markup, or rich text in the body. For example, if an external vendor is having an issue, I may want to provide a link to their site for more information, or I might want to emphasize information in the announcement.
3) Change the default announcement color. (As stated elsewhere)
4) Post critical announcements on the login page. (As stated elsewhere)