Search and indexing: Thunderbird 3 indexing is fast and very good. The interface for searching and drilling your mailbox is fantastic, very well done and fast. Check the screenshot.
Tabs: I like to be able to read messages in different tabs. Lightning calendar and tasks conveniently open in a tab and it’s also possible to run Google Wave in one. This shows the power under the hood of this client: it has the potential to become a messaging hub for all services. It’s annoying that Addressbook and compose new mail open in a new window instead of tab though.
Add-ons: just like Firefox, there are many ways to add functionalities to Thunderbird. The first add-on I installed is Funambol in order to test it and help its development. Then ThunderBrowse, in order to avoid opening Firefox only to check a link in an email and EnigMail to add GPG/PGP support. Nothing mind-blowing yet, but I hope somebody will develop a replacement of the Addressbook with more ‘social’ features. A topic for another post 🙂
Bonus reason: finally! There is an “Archive” button: once you’re done with a message or a thread, and you don’t want to delete it, you hit a button and the message goes into the archives (archiving criteria can be specified). A neat solution for Capo’s problem 🙂
The Addressbook, on the other hand, is pretty lame. It doesn’t contain enough fields, it still has space of a ‘pager number’ (anybody still using them?), it opens in a window and not in a tab. I hope it’ll improve in next version.
I’m still using Evolution as my main source of data, especially address book and calendar. Even if Funambol add-on for Thunderbird 3 works decently, it’s still unstable and it has other small issues (if you want to help, Funambol offers Code Sniper grants).