What's New In Outlook 2010?
It seems like just yesterday that we were welcoming Outlook 2007 to the world but here we are on the cusp of the next version of Outlook. On the surface Outlook 2010 looks more evolutionary than revolutionary but under the hood there is a lot more than meets the eye.
Hence the Ribbon!
Outlook 2007 did not make as extensive a use of the Ribbon interface that was found in (most of) the rest of the Office 2007 suite. The "Explorer View" (which is the view you use when you're viewing your Inbox list of e-mails, your Calendar, etc.) still used essentially the same interface that Outlook 2003 used (with a few minor changes). The Ribbon shows up in Outlook 2007 in the "Inspector View" which is what you use when you actually open an e-mail item or an appointment item and work with that individual item.
With Outlook 2010 the Ribbon - the new and improved Ribbon - now appears throughout the product.
The new Ribbon is just a bit different from the old one. The first big difference you'll notice is that the Office Button (AKA "The Pizza Button") at the top left corner of the screen is gone and has been replaced by "Backstage" which looks a little more conventional. Basically there is now a "File" tab on the left end of the Ribbon and clicking it gives you all of the options you used to find on the Office Button, plus just a bit more. Things like printing, account settings, options, Automatic replies and other such now appear on the Backstage menu.

The rest of the Outlook Explorer has a clean look with the new Ribbon providing items on the Home, Send/Receive, Folder, View and Add-Ins tabs.

My Ribbon above looks slightly different because I have the "Developer" tab turned on. You might not.
Quick Steps
One feature that attorneys will like in the new Outlook are the Quick Steps, which you can see above. Essentially Quick Steps are like mini-macros that you can create to automate steps you take often. 
You can use Quick Steps to automatically start a reply to a mesage, set a category, move a message to a subfolder, flag it in some way...or maybe a combination of all of the above.
This can be a great way to automate repetetive tasks and make sure you don't forget a step you wanted to take.
Be Part of the Conversation
Outlook 2010 is all about the conversation. Messages can be shown grouped together into threads based on the conversation and even messages from other folders (like Sent Items) can be shown as part of the conversation. This is great because it makes it easier to follow the flow of the exchange of messages, especially if there are several parties involved. But also because it reduces the need to do one of the most inelegant things that too many lawyers do with e-mail: CC themselves on every message they send.
The reason they do that is because they want to keep their response grouped with the received messages in the conversation. But it's inelegant and wasteful. Now with Conversation View in Outlook 2010 they don't have to - messages in Sent Items that are part of the same conversation will be displayed alongside the received messages WITHOUT needed to have a separate copy of the message in the folder.
Outlook 2010 also gives you the ability to ignore certain conversations. If you're on a busy e-mail list, for example, you may find that you get a lot of messages that you really don't care about. If you want to help keep your mailbox cleaner you can choose to Ignore certain conversations and Outlook 2010 will not only delete all of the messages you already have in that conversation but will automatically delete all future messages in that conversation (at least for a few weeks).
Already downloaded a lot of messages in a conversation and want to clean up your mailbox? You're in luck - Outlook 2010 has a "Clean Up" utility that will go thru your conversation or folder (or folder and subfolders) and clean up redundant messages to keep things lean and tight.
Let's Get Social
One of the most popular add-ins for Outlook 2007 has been Xobni - a tool that shows you all sorts of statistical information about your messages and helps you find messages folks have sent you. Some of that functionality will now be done by the Outlook Social Connector (OSC).

The Outlook Social Connector appears at the bottom of the Reading Pane in Outlook and gives you a place to see previous messages from the same sender, messages containing attachments, appointments linked to that contact and other such information. There is also an API available that promises to have add-ins written by other vendors - for instance there is now a "LinkedIn" provider that will integrate LinkedIn information for your Contacts. Keep in mind...this is all still Beta so be prepared for possible bugs. I can only assume that Facebook and other vendors will be doing the same.
Too bad we can't have this in Outlook 2003 or 2007 you're probably saying. Well...good news, you can! Yes, the link above will work for Outlook 2003, 2007 or 2010 (32-bit only for the moment). Again, it's a beta, so keep that in mind.
Ignore Conversation
If you're on a busy mailing list, or if the folks at your office just abuse the "Reply to All" feature, you may find your Inbox overflowing with conversations you really don't care about. In the past that meant a lot of persistent deleting. In Outlook 2010, however, Outlook can do the deleting for you. By selecting a message in the conversation and clicking "Ignore" on the toolbar (or pressing CTRL+Delete) Outlook 2010 will automatically delete all of the messages in that conversation and will direct any further messages in that conversation to the deleted items folder automatically. Handy way to keep your Inbox a little cleaner.
Other New and Improved Features
- Integrated RSS Aggregator is faster and more reliable now. In 2007 it was rudimentary and a little troubled on occasion. In 2010 it's more reliable.
- 32-bit and 64-bit versions. Finally a version of Microsoft Office that takes advantage of today's more powerful hardware! Naturally you do have to have a 64-bit operating system in order to use the 64-bit version of Office.
- E-mail Attachment Previewing - view Word, PowerPoint and Excel files without having to actually open them.