A post on the The Rumbling Edge, Mozilla’s site for cutting-edge developments, lists the features and improvements 
that will appear in Thunderbird 2.0 Alpha 1.0, which has not yet been released.
Top of the list are custom Folder Pane Views for things like favorites, unread and recently used email. You will also be able to tag messages, and open multiple messages in individual tabs.
The new version will offer built-in notification of new email, which may look something like this:
You will be able to get pop-up summaries of the contents in folders.
Security will be enhanced with an improved Bayesian algorithm and better protection from Phishing.
Lastly, it will offer support for “find as you type” filtering.
The post lists all the many more fixes and improvements in detail, including twelve Mac-specific issues.
I see nothing to tempt me away from Mail.app, but it is interesting to see how Thunderbird is developing.
I hope they change the icon – it’s really ugly!! They also need to improve the search algorithm – I find it unreliable.
How about a decent Mac UI experience? :) If it felt like a Mac UI, I think it would replace Mail.app for me, but I can’t abide the XPToolkit experience, it’s just so far below the usual Mac UI.
Thunderbird needs Spotlight integration or vice versa.
It´s the best mail app for PCs and has GoogleDesktop integration….it could also be the successor to Apple Mai (especially since it has become more and more ugly)…but not without Spotlight.
@Jeremey:
Maybe you should have a look at the following links.
http://www.hawkwings.net/2006/05/02/tiger-mail-skin-for-thunderbird-updated/
http://www.hawkwings.net/2006/03/19/a-mailapp-style-thunderbird-icon/
For me, the showstopper for Thunderbird is simple: Direct integration of Address Book. If it used Address Book, I’d be running it today. (’sync’ with address book would not count. Anything less than native use is not useful.)
I should clarify my aversion to sync: Address Book on Mac OS X is a system-wide framework used by multiple applications, and it’s much less useful if you can’t rely on the contents of the Address Book to be up to date at any given time.
@Chris: Have you seen this attempt to get Address Book integration up and running?
I haven’t tried myself, but others say that it works.
@ToPPi: thanks. I’ve used the skin and it was something of an improvement, but of course it’s really just making the experience look nicer without modifying behavior. What I would hope for would be something like Camino which implements the full set of Thunderbird features and options but with a native Cocoa UI. But I have to believe that significant improvements could be made in the XPToolkit so that Mac UIs look like modern Mac UIs (not carbon apps from years ago).
What about being able to rename attachments before saving to the desktop