Another large area where mailnews could use a pluggable interface is storage. In my last post, I discussed my thoughts on storage. To clarify one point: I am not against pluggable storage APIs or even implementations of most of those presented; it is just my opinion that mbox should probably be the default in lieu of a better choice.
A third area is the account manager. As far as I can tell, the only thing everyone can agree upon is that it does not work well as it stands. Try creating a new account type and you'll see what I mean. Killing RDF will help this in part, but the current consensus is to hold off a rewrite in this area until post-Thunderbird 3.
To keep this post short, I'll omit details in the other areas where we could use pluggability. The view pane could use it at least in part (see this site for more information). Supporting synchronization with mobile devices (several bugs exist here) would need one, if we want to go beyond palmsync.