I took a swing at liking Mail.app. Here's my short list of what it needs: keyboard navigation (I don't use the message preview view), to check my mail for real (I thought it was quiet afternoon, until I launched Thunderbird), a junk filter that works (and moves junk from inbox), and not to download and index all of my sub-folders (I know how to search existing folders! it's been sorted!).
And then I went back to Thunderbird, all of my new mail showed up, all of my junk mail vanished, and the navigation was easy. *happy sigh* Sure, Mail.app is prettier, but basic functionality trumps that.