The update looks to include a number of features that you can find in the Kindles, K4PC, and the other apps. For example, the Kindle Fire will be getting support for Kindle Print Replica, Amazon's PDF format.
It will also get the X-Ray book extras that shipped on the Kindle Touch and K4. These extras are drawn from user provided info on Shelfari and include supplemental material about the book, including descriptions of characters, a glossary of common terms used in the book, information on the authors and common locations referenced in the book.
Silk, the web browser that time should have forgotten, will be getting a new reading mode. It sounds like it's going to be able to pull say the text of a blog post and show it in a separate window (without the advertising or other junk found on the page). The webkit based browser on the Kindle, as well as other browsers have a similar option. The one on the iPad is particularly nice.
And the KF will also be getting the sharing options which will let readers Tweet and post to Fcebook their favorite passages and notes from their books directly from Kindle Fire, without even leaving the book.
Last but not least, the new Kindle Fire update will also add support for Kindle Cloud. Got your own ebook collection? Amazon will now back them up to their servers from the KF, including whatever notes you may have made.
All in all, it looks like the Kindle Fire is about to get a lot more useful. This will be a major change in the abilities that the KF shipped with 6 months ago.