Luckily, Mozilla hasn't made the mistake of fixing what ain't broke. Thunderbird 2.0 will still include free extensions to add functionality and themes to change the look and feel at your whim. The spam filter still catches most spam before it clogs your inbox, and it still includes a terrific search function and built-in RSS. Mozilla also has made some welcome changes without breaking anything in the process, but there are features that could still use some work.
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I downloaded the installation program and installed without an issue. The install found all my mail, folders, and settings and imported them seamlessly into the new version without prompting. My Thunderbird 1.5 extensions and themes didn't have version 2.0 equivalents, but that's to be expected when the beta was only recently released.
There is a distinct difference in the look and feel of the application. Mozilla has updated the toolbar, brightened and changed the icons, and made the interface generally more pleasant and readable. For instance, the icon that lets you flag your e-mail messages is now a gold star, which is much more visible than the small red flag in the older version.