Among the numerous subtle improvements Apple included in Snow Leopard:

Single-click anywhere in a previous reply to highlight it…

…et voilà!

While you’re composing a reply, clicking a paragraph in a previous reply highlights it (and any replies nested below it) allowing you to remove the whole block with one more click. It does leave two lines you have to delete manually, but it’s still pretty handy for cleaning up long threads or removing part of a conversation.

Update: It’s been brought to my attention that this is not new in Snow Leopard, and that it requires rich text formatting in the original and in the reply to work. So it’s not quite as useful as it first seemed.