Here are some miscellaneous thoughts on Thunderbird:
1) I like this Nostalgy add-on that Ira told me about. Basically, it makes TB more friendly for those who would prefer to control it through the keyboard, rather than having to click and drag. Here's an example. If you have a ton of folders like I do, and you want to file mail into those folders as you are done with it, Nostalgy allows you to hit the 's' key and start to type the name of the folder where you want your message to go. It will do an auto-complete for you and you can just hit enter to move the message there. This also works with copying and tagging messages.
If you're into using the mouse, this add-on is not for you. But, I would say it's perfect for someone who is a....former elm user. (I'm looking at you, Al!)
2) So, the other day one of my cats (JL, I know you're wondering which cat; it was Tiger Woods-Mustazza) walked across my keyboard and f-ed up inbox. All of my messages got grouped into categories based on when they were received. Moreover, they were put into these collapsible containers that would close from time to time and hide my mail - ugh! Somehow, I got everything to go back to normal.
A couple of days ago I clicked this icon of a swirly thing and all of my mail got grouped into conversations, like in GMail - and, again with the collapsible, nested categories! Again, I could not figure out how to turn this off.
Thanks to MacDerm, I now know how to get out of these situations when they arise: just click one of the headers that sorts your mail in your inbox - i.e. click on "Date" to sort your mail by the date. This causes said collapsible categories and threading to vanish. Thank God.