The new version of Mail that ships with Lion is certainly an upgrade that’s worth it’s weight. One of the cool features of Mail now is the ability to group related email messages together and to remove a lot of the duplicate content when people quote part of an email back to you.
By default though you have to click the “show related messages” icon at the top Mail’s new interface.
If you have a look in the System Preferences of Mail you will see there is a new option to turn this on all the time.
Check the screen shot below for the location of this setting.