I won't bore you with all the email features since we all know it's an email client that supports IMAP and POP and has a robust address book, etc., etc. One new feature you may not know of is its support for chat networks.
As you can see, the new chat feature supports IRC, Google Talk, and Twitter among others. I tried the Twitter client. It's not a full-fledged feature-rich one, but you can see your timeline, get real-time updates, and send out tweets.
Another great use of Tenfourbird is as an RSS and Usenet reader. They work in a similar way. You subscribe to feeds or newsgroups and they're displayed in the folder pane as folders with all your recent and unread messages. Very convenient for reading through a lot of headlines quickly.
Then there's the Lightning add-on that Tenfourbird is bundled with. When you enable it in the Add-ons tab, it gives you a calendar/task manager, and you can also sync it with your Google calendar by installing the Provider for Google Calendar add-on. Here's me demoing the events manager. As you can see, I'm not much of a calendar person.
One final new feature is called FileLink, where you can upload a large attachment to an online storage service and send the recipient a download link instead of emailing them the whole attachment. This is useful when going up against email size limits your server imposes, and it's all automatic once you set it up in the Attachments preferences. By default, you can set it up with your accounts at Box, Ubuntu One, or YouSendIt, and you can add Dropbox with the Dropbox for FileLink add-on.
Also on the subject of add-ons, Tenfourbird's developer provides versions of Enigmail specifically compiled for PowerPC, so you can have support for OpenPGP and have some semblance of privacy.
The only downside is there's no easy way to sync Tenfourbird's data across all your computers. You can use a file syncing utility like Unison to sync your Thunderbird profile folders, but you have to remember to actually sync them after each use. You can sync your online calendars to a degree, and if you only have IMAP email accounts then no problem there, but I have a mix of IMAP and POP accounts. On my secondary computer I have Icedove, so I set my POP account there to leave messages on the server so I can still download them onto my primary computer even if they're already read. That's not really syncing, but it keeps things more or less in order.
All in all, Tenfourbird gets five out of five stars.