Unless you are tethered to Exchange, there really is no reason to use Outlook. It's big, bloated, and, with the 2007 release, ridiculously slow.
Still, there's always been one aspect to how it threads email messages that I liked. When you viewed messages by conversation, it would bubble threads to the top when new messages for that thread arrived. This had the effect of letting me see the freshest threads at the top of the stack.
Thunderbird is a far more capable email client but I've always disliked the fact that it orders the threads by the date of the first message in each thread. Unless you are very organized and keep your inbox clean, you can easily lose messages when they are part of a very old thread.
It turns out that Thunderbird can sort messages the same way as Outlook. The key is to tell each view to sort by date instead of order received. Sorting by date will sort by the date of the last message in each thread while sorting by order received will be based on the date of the first message in each thread. You can then choose ascending or descending on that same sort menu. Now if I can just get a stable Outlook <-> Google calendar sync program running I will have no reason to run Outlook 2007 at all.