In prioritized order (for me):

bug 187578 - From-field when replying to message in "local folders" should be "To"-field of original

bug 228980 - Identity selection based on recipients should be more correct

bug 30057 - Use one Local Mail tree for all POP3 accounts

bug 46041 - Multiple accounts without multiple folders (global inbox, etc.)

bug ??? - Can't insert other signatures for a message being composed by simly goin to menu Insert -> Signatures -> ...

bug ??? - No button and keyboard shortcut to perform send&receive of _all_ mail accounts that have "Include this account when receiving mail or synchronizing" checkbox checked in their config. Send/Recv button with dropdown of: "Send and Receive All" (default), "Receive All", "Send All", separator, individual accounts list...

bug ??? - Show recipient&sender e-mail address and/or name instead of just the name in messages list/preview panel

bug 22112 - UI: Icons should show state of mail

bug 43278 - Crossposts (same Message-ID) not marked as read in other groups

bug 33045 - Newsgroups: Show column for "size" (in KB), not "lines"

bug 171907 - drag & drop an eml file into a mailbox folder

bug 193281 - Ability to import e-mail messages from .eml text files

bug 44863 - UI for multiple identities per account

bug ??? - Display number of selected messages

bug ??? - Threads can't be collapsed by default

bug ??? - Marking message as unread doesn't change icon back to unread (fixed in TB 0.6 but not in the suite)

bug ??? - Saving a message to drafts folder doesn't mark it as unread

bug ??? - Select which folder to open on startup

Crushing those bugs would make MailNews/TB heavily superior to OE.

Read 'em & weep ;P