Some of the features I like in Thunderbird:
1) Spam filters work wonders, it just takes a couple of weeks/months of training (depends on the amounts of spam you receive), and then it'll do a great job detecting spam.
2) Privacy options, like not loading remote images, or showing all emails as plain text.
3) You can configure it to leave large messages on server if you use POP3, to avoid downloading large messages, later you can decide whether to download or delete the message depending on the sender/subject...
4) Great search and filtering options.
5) Better integration with the greatest browser out there, Firefox
6) A wide variety of extensions and themes.
7) Free and Open Source (which means faster updates and patches).
...
And a lot more, these are only a few things I can think of now.
For calendar and notes, check out Mozilla Sunbird:
http://www.mozilla.org/projects/calendar/sunbird.html