What I noticed about Thunderbird in comparison with Outlook:

Faster, slicker UI
Although Thunderbird doesn’t look as great as Outlook it certainly has got a nice interface which is customizable enough.
The Interface itself is also a lot faster than Outlook`s, which I find more important than grafical splender.
Also the collapsable headerview above emails and information bars above the email (eg. a bar showing that thunderbird thinks the email or spam or that thunderbird is blocking images) are a nice addition and are worked out better than in Outlook containing them in a limited way.
Also thunderbird seems to interface directly with firefox letting it show html emails a lot faster than Outlook does.

Configuration
In the beginning the configuration was rather confusing. Although this also applied to the configuration with Outlook when I first tried it out. I still think that the account configuration interface could have been a lot more intuitive.

Outgoing mail
Thunderbird requires and actually uses only one smtp server for all your mail accounts. Although you can specify more it doesn’t seem to be able to specify which Smtp server to use to sent the actual mail. This is a shame for espacially when sending important emails it is more trustworthy if the email can be traced back to the smtp server of the original domain. Also some smtp servers don’t allow seemingly spoofed `from` fields.

Extensions
Thunderbird seems to have a very similar extension system as Firefox which I can appreciate. There are some very usefull extensions like Enigmail which is a front end for GPG for Assymetric email encryption. It doesn’t only manage your encrypted emails, it also got a very good interface for all kinds of GPG operations like all kinds of key managment.

Everybody should give thunderbird a try, it’s a great toy .