Thunderbird 3 improved the filter story dramatically here; the next two biggest itches are the complete inanity that is news URIs (too many bugs to count), and the venerable old crossposts bug. I still contend that the latter would best be served by per-account database functionality; in any case, it does require some database changes to work properly. I doubt I'll find time to look at that bug in particular this year.
The URI issues are more tractable, but I don't think I'll find time to hit them for 3.1; in any case, I now consider them to be the highest priority news bugs. So, to anyone with time on their hands: feel free to take one or two of these and start fixing. You'll get much kudos from Thunderbird Usenet users.
Other various "nice-to-haves" on my list: fixing subscribe, cleaning up some of the gunk in the code, adding support for RFC 3977 CAPABILITIES, possibly changing how news:a.group URIs work (open the folder view, not necessarily subscribing to them), among others. Combine-and-decode also falls under this list, but it's a lot less tractable than some of the other stuff.
jshydra could use some more love: I hope to be able to be able to get a converter to a more natural AST working by the end of the year, as well as an automated test suite to verify correctness whenever I change m-c versions. I've also been working on-and-off on getting symbols for DXR via MSVC, which should hopefully also be finished this year.
Other Mozilla/Mailnews work
I've yet to see a roadmap for the address book in 3.1 and later, so I don't know what I'll be doing for the address book in this upcoming year. I expect, though, that I won't do anything near the scale of what I did for bug 413260. De-RDF and de-morkification are another two things I'd like to see worked on that I don't expect to get to this next year as well.
Time to see how much I'll actually get done this year!