Second is lack of support for the Keychain. I will not use any app that has it's own password storage function. I want to know where my passwords are and to be able to manage them easily. I have the same problem with Firefox and will not use it either as a result. I don't understand why developers do this.

When using my .Mac account in IMAP mode, it did not handle my server folders correctly, nor was I able to easily set up to send from my .Mac email aliases as I am in Mail.

And finally Thunderbird does not take advantage of the real-time system wide spell checker, which I have come to rely on.

It is a shame because some people clearly worked really hard on Thunderbird and it does have some promise. Saved searches are nice, and RSS is neat, but we'll be getting those with Tiger Mail and Safari respectively in a few months plus a lot more. This app has the look of an OSX app, but under the skin the feel of a PC port.

I can see how Windows and Linux users would consider this a viable client, but for me at least I'll have to wait for 2.0 and try again.

Perhaps in organizations with heterogenous platforms there will be an appeal for an app that is standardized across all the platforms, but that approach has its perils too. You can end up with the jack-of-all-trades but master of none scenario, not to mention that a security flaw can grow more easily into a huge problem if all the systems are running the same software.