One thing I didn’t see mentioned was Address Book Integration/enhancement. As I’ve suggested before, the Address Book has a nice feel to it, but it misses some features. And a key part of any PIM is the address book. For Calendar to really rock with Thunderbird, it’s going to need to integrate nicely with the address book. IMHO Address Book 2.0 reloaded should be included with this.
Hopefully Palm/PDA/Cell phone synchronization methods will follow shortly. I think #1 is getting Palm Sync going. Especially since Thunderbird already supports it for addresses and email. That way Palm users are 100% covered at this point.
I’ve started working in iPod support for Thunderbird, and do plan for Sunbird to be supported as well.