From the "Mozilla does email? really?" files:

At long last, Mozilla Messaging has put out the first Beta release for the Thunderbird 3 open source email client. Lots of new stuff here that users will notice. According to Mozilla among the new features are a tab interface for mail, improved message reader view, better address
book interface and improved import of mail from other Mail clients.

Beta 1 is not however a feature freeze for Thunderbird 3 and Mozilla Messaging chief Dan Ascher hints that much more is to come before Thunderbird 3 hits its final release.

"Unlike the traditional definition of a beta release, we're definitely not done making feature changes, including some pretty significant feature work that we expect will be integrated in Thunderbird 3 in later beta releases, some features that will live as optional add-ons, and some experiments which may end up in later releases of Thunderbird or not, depending on the result of the experiments," Ascher blogged.

For me personally the item that I'm most interested is something that actually isn't directly part of Thunderbird but rather is an add-on. Thunderbird on its own lacks a Calendar which means it can't directly compete against Microsoft Outlook.

The Mozilla Lightning add-on is an effort to add a calendar to Thunderbird (or Firefox too for that matter if you wanted too).  By Ascher's own admission the Lightning project isn't done yet with it's work, but so far in his view things are looking great.