The new taskbar in Windows 7 Beta 1 has changed quite a bit since the familiar Vista and XP taskbar that we are used to.
There are a number of new features and changes that I find very useful with the new taskbar. Here are some of my experiences with the new taskbar so far.
The overall look

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Normal icons and taskbar buttons set to "Combine when full".
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Normal icons and taskbar buttons set to "Always combine, hide labels".
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Small icons and taskbar buttons set to "Combine when full".
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Small icons and taskbar buttons set to "Always combine, hide labels".
When using "Combine when full" the pinned icons of applications that aren't running will displayed with the icon alone.
When using "Always combine, hide labels" the icons of the running applications will look almost the same as the other icons. The running icons have a border and will display a highlight color when you move the mouse over them.
Grouped applications (See the Live Messenger icon above) result in one or two edges on the right side of the icon to signal multiple running applications. This seems to works very well and it's easy to see that a taskbar button is holding multiple instances of an application. When right clicking on a grouped taskbar button the close message will state "Close all windows" instead of the normal "Close window".

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Four instances of Paint running and grouped togheter, looking almost like one big taskbar button.
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The first application is selected and has focus.
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The mouse is hovering the second running application. (Notice that the highlight is within the group, and not only the single taskbar button.)
The highlight color of a taskbar button is decided from the most intense color in the application icon. (IE is blue, firefox is brown, Excel is green, etc..)
This feature might look like a "just eye candy" feature, but you might notice after a while that you're both quicker and more accurate when selecting a taskbar button.
The system tray

In the system tray there are two major changes from Vista. To the right of the time & date is a "Show desktop" button that will minimize all programs if clicked and make all programs transparent if you hold the mouse over.
Second is the refreshed "hide inactive icons" from XP and Vista. This function now let's the user decide what tray icons should be displayed in the tray, what icons can show notifications and who should stay hidden.
The icons in the system tray can be arranged as you like, just like the taskbar.
Finally you can have exactly the icons you want in the system tray :)
The taskbar button "blinking"

If you use Live Messenger or any other application that use the taskbar button to signal when a new message is received, you are most likely used to that taskbar buttons blink from time to time.
It's an OK way to tell the user there's a new message waiting that needs to be read, but not always the best solution when you're busy with something else.
In Windows 7 the taskbar button will blink 3 quick times (just like in XP or Vista), 2 times very slow and then finally stay in the orange highlight color until the application has focus.
See video of the new blinking here...
Progress displayed in the taskbar button

When you transfer a file between two locations in Windows 7 you will notice that the taskbar button for the file transfer will also display the progress of the transfer.
This currently only works on File Transfers, but I'm hoping this will be included in most applications that use progress bars in the future.
Jump Lists

The Jump lists (Right click a taskbar button) are new in Windows 7 and include three standard commands and a list of custom commands that the application has chosen. With Notepad the list are the recent documents, in Paint it's recent images and in Live Messenger it's a set of basic commands like status, sign out, email inbox and Windows Live profile.
The three standard commands are "Close window" ("Close all windows" if it's a grouped taskbar button), pin or unpin this program to the taskbar and a link to the application itself. (Same as shift + clicking the taskbar button)
Tips & Tricks
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Windows key + 1-9 will launch a new instance of the pinned application on the taskbar. (Similar to Vista's quick launch)
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Windows key + Up key will maximize the application and the Down key will minimize it.
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Windows key + Left or Right key will snap the application to left or right with 50% of the screens width.
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Hold shift down + left click a running application will start a new instance of it.
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Drag a window to the top will maximize the application.
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Drag a window to either of the sides will make the application take 100% height and 50% of the width of the screen. (Good when comparing two documents)
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Taskbar buttons can be moved around on the taskbar.
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Left click on a grouped taskbar button to show the thumbnail preview instead of waiting for the "mouse over" effect to trigger. (This is a must if you have set the Taskbar Button option to "Always combine, hide labels".)
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Windows key + Plus (+) to zoom in on your mouse cursor. Use Windows key + Minus (-) to zoom out.
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Windows key + P to select display mode (display, display + projector, display and projector or projector only).
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Shake a window left and right to minimize all other running applications. Shake it again to restore them.
Remote Server Administration Tools
RSAT for Windows 7 can be found at http://www.microsoft.com/downloads/details.aspx?FamilyID=82516c35-c7dc-4652-b2ea-2df99ea83dbb&displaylang=en
X86 installer: http://download.microsoft.com/download/A/D/4/AD4D3903-E06D-456D-AED4-D53895D2C1A9/Windows6.1-KB958830-x86.msu
x64 installer: http://download.microsoft.com/download/A/D/4/AD4D3903-E06D-456D-AED4-D53895D2C1A9/Windows6.1-KB958830-x64.msu