Straight from Microsoft, people! Check out the press release:

Windows Mobile 6.1 — New Features at a Glance

GETTING STARTED

Getting Started Center

The Getting Started center provides a way of helping people perform the primary tasks the phone is used for during the first 10 days. Getting Started shows people how to perform important setup tasks, including date and time, personal and work e-mail, security passwords, Bluetooth headsets, backgrounds and ring tones, as well as the ability to transfer music from the PC.

PERSONALIZATION

Sliding Panel Home Screen

Windows Mobile 6.1 gives people an enhanced and simplified way to retrieve their information. One of the first changes users will notice is the new home screen user interface. The Sliding Panel plug-in offers quick, at-a-glance information of the clock, notifications (including voice mail, missed calls and text messages), e-mail, appointments, music and, optionally, Windows Live for Windows Mobile. In addition, once the Start menu button is pressed, people will notice it is now reordered to display the most recently used programs and applications at the top, enabling quicker access. The Most Recently Used Applications view can be turned off from the home screen settings panel.

 
ENHANCEMENTS TO MESSAGING

 
Threaded SMS

Windows Mobile 6.1 allows people to view a series of short message service (SMS) threads in one view, cutting down on search time and providing one view of a conversation’s history. Entries also show that contact details and contact names are hyperlinked, allowing people to respond immediately via text, phone or e-mail.

Multiselect Functionality

In Messaging, people will now be able to select multiple e-mail or text messages from the List view and enable various bulk actions: delete, move, mark read or unread, and flag messages. In the Contacts view, people can select or select all to delete multiple contacts at once. This feature will also be supported for multiple messages or contacts within a smart filter search.

Direct Push Technology

The over-the-air synchronization process in Windows Mobile 6.1-based devices has been improved further with Exchange Server 2007 Service Pack 1 (SP1). The synchronization architecture has been redesigned to reduce bandwidth by up to 60 percent and improve battery life by up to 33 percent (as tested on internal test devices between Windows Mobile 6 and Windows Mobile 6.1).

PRODUCTIVITY

Cut, Copy and Paste

With Windows Mobile 6.1 non-touch screen devices, people now have the ability to select, cut, copy and paste text, significantly enhancing the user experience. This functionality will be available for composing

e-mail messages in Microsoft Office Outlook Mobile, for text messaging and for working with editable fields in Internet Explorer Mobile. This feature has been previously available in Windows Mobile touch screen phones.

Task Manager Application

Windows Mobile 6.1 adds a Task Manager to help people better manage their device’s memory and battery life by allowing them to shut down and switch applications and programs. Users are also able to view general performance data regarding device memory and CPU use. For example, if a person wants to listen to music using Windows Media Player Mobile while running other applications, a message may appear notifying the user of insufficient memory to launch the player. Using Task Manager, that user can quickly navigate to the list of applications running on his or her device, exit unnecessary applications and free up memory.

This feature is anticipated to help reduce mobile operator support costs as devices become easier to troubleshoot.

Live Search for Windows Mobile

Live Search is an optional in-ROM application for device-makers to include in Windows Mobile 6.1-based devices. Live Search for Windows Mobile helps people search for and find destinations and content. Examples include finding restaurants and movie times, checking traffic, getting directions and finding the cheapest gas prices in a city.

Improvements to Internet Explorer Mobile

Internet Explorer Mobile enhances a person’s ability to navigate a particular page on the Web. People can select a portion of a page and return to the standard viewing size to see the page up close. Once a person has accessed a particular Web page, he or she can move the selection tool over the section and view it close up.

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Available programs, features and functionality vary by device, Windows Mobile operating system version, and version of Exchange Server used. Connectivity and synchronization may require separately purchased equipment and/or wireless products (e.g., Wi-Fi card, network software, server hardware, and/or redirector software). Service plans are required for Internet, Wi-Fi and phone access. Features and performance may vary by service provider and are subject to network limitations. See device manufacturer, service provider and/or corporate IT department for details.

 I like the homescreen, but it doesn't seem that the new interface extends throughout the OS. Anyone?

It's where it makes obvious sense, at least on the version I'm running.  The major differences are on the home screen.

Will there be a release for the Treo 750?

eNurse

 Check this thread for the list of MOs and manufacturers who have announced support.

Flash Lite will be very nice too....I'm tired of having to load vTap on every phone I have just to watch YouTube vids.

Keep in mind that the new Sliding Panels UI is designed for Windows Mobile Standard phones only.  Professional phones do not receive such a makeover.

This is the main comment I have had from customers.. Why does standard look so awesome and professional looks....well the same as 6 really.   Its a weird one..

Anyone notice how the ever-popular carosel interface on the samsung i607s is now taking a major part in the popularity of 6.1 standard? 

Yeah, this is quite puzzeling to me too. Why only make changes to the Standard version and not the Professional as well?
So as far as I know, the only interface change was the threaded SMS on the Professional. Other than that, it's the same old WM6.

Ahmed Eltawil:

Yeah, this is quite puzzeling to me too. Why only make changes to the Standard version and not the Professional as well?
So as far as I know, the only interface change was the threaded SMS on the Professional. Other than that, it's the same old WM6.

 You also get the zooming browser, which works great one handed with the hardware buttons.  The new task manager also works great for one-handed usage. The threaded SMS, Domain Enroll, and Managed Programs compatibility are also there, along with the "Getting Started" guide.   Here's more details. 

Also there's some nice little improvements.  For example, adding bluetooth devices automatically attempts common PIN numbers, thus making pairing a lot easier.  And in the Messaging app, if you go to change settings for an account, you don't have to go through the whole wizard again.  You can automatically jump to the account setup, send/recieve schedule, or download size settings sections.

Well I guess you wouldn't notice the improvement until you dig into the OS. It would be nice to see a new improved homescreen on the 6.1 Professional version as well. Why not have the sliding panels for the professional too? It would be nice just to swipe my finger across the screen to flip through the panels.

Maybe Windows Mobile 7 is actually not so far and that's why Microsoft didn't bother making a lot of changes to the UI on the Professional version.