Welcome to a fresh look for the new Yahoo! Mail

Based upon user feedback, we have made Yahoo! Mail even better than before by streamlining the entire look and adding new functionality to provide a better email experience.

• Less Advertisement
  We've removed all the ads on the left hand side of the page.
• Increased font
  To improve the readability of email, we've optimized the font size.
• Simplified Email Filtering
  Easily filter your messages between your Contacts and Connections under the "View" menu.
• Reading pane now easier to resize/minimize
  We removed the "X" in the Reading Pane making it easier to resize/minimize this pane. To minimize, click the down arrows in the middle of the reading pane bar. To resize, mouse anywhere on the bar (except on the down arrows) and drag the bar up or down to any location that suits your needs.
• Improved attachment management
  By setting up the new attachment tool, you are able to select multiple files at once (including an entire folder), see thumbnails of photos you upload, rotate photos, and the ability to send more photos quicker - click here for more details. If you do not want to try the new tool, you'll still be able to select multiple files at once and see the upload progress of each file
• Applications Box below your Folders
  All of the useful/productive applications you're used to using have now been nicely organized in the bottom left hand side.
• Calendar and Notepad now grouped with Applications
  We've tucked some of our favorite Yahoo! applications under the new Application Box.
• Select Messages in View
  In addition to selecting all of your messages in your Inbox, you can now select just those in your view. Click on the "actions" button and choose "Select Emails in View".
• Improved tab management
  After you send an email, the send confirmation tab will automatically close after you've viewed it getting out of your way. You can still go to your "sent" folder to view any emails that you have sent.
• Message size column turned off
  To clean up your inbox, we've removed the message size column. If this is important to you, just do the following to enable the size column again:
  1. Click on the View button at the top right of the page.
  2. From the drop down menu, under "Show:" select "Size Column"
• Refreshed color options
  With a fresh look come fresh, new colors! We hope you enjoy them as much as we had fun creating them.
• Moved "Print" button under Actions Menu
  We moved the "Print" button under the Actions Menu in the inbox/folder view.
• "Mail Classic" button moved under Options menu
  We have simplified the Mail header and still provide you the option to switch back to Mail Classic if you want. To do so, click on "Options' on the right-hand side and select "Switch Yahoo! Mail Classic…"

http://help.yahoo.com/l/us/yahoo/mail/yahoomail/context/context-20.html