If case you haven't used Manage Your Kindle before (and I'm willing to bet most Kindle users haven't) it allows you to delete ebooks, send them to particular Kindles and even download a book to your computer and transfer it to your Kindle via USB.
Here's what it used to look like (and still does in Chrome for me):
The design isn't very exciting, but that's not my main issue with the old MYK site. You couldn't perform actions on multiple books, so if you wanted to delete 3 books from your library you had to click on the drop down for each and then click delete. I don't have time for 6 clicks!
The new version makes that a thing of the past, and looks much nicer (at the moment I'm only seeing this in Safari):
A lovely gird of your book covers is displayed by default. Click on a book and it gets a little green check mark. Click on another one, and another check mark appears. Then you can click on one of the action buttons and have that action apply to all the selected items. Magic:
Amazon has also made it easier to find details about your Kindles by displaying all of your them along the top of the page. Clicking on one Kindle allows you to deregister it (if you want to have someone else use it with their Amazon account), your Kindle's email address (you did know you can email documents to your Kindle, right?), and the type and serial number.
It is also now much easier to turn off Special Offers (i.e. ads) on your Kindles that sport them, just by clicking a link and paying a few bucks:
Well done, Amazon!