The things that bother me about Thunderbird on OSX are certainly many, but I can come up with one above all others that totally kills me: the lack of integration with the Apple address book. Nothing more than this illustrates the source of Tantek’s fervor for wanting data portability and his resultant hope in microformats.
Think about it. If Thunderbird stored hCards, and Address Book.app read hCards (or used them as its storage format), there’d be no problem.
One format to rule them all: XHTML! Best of all, you could use Spotlight, Applescript, and whatever other Mac-centric technologies on this data as well. No weird one-off formats that nothing else supports, no conversion, no special readers or parsers… and you could upload your address book and view it on the web… anywhere.