Has enough UI polish to make day-to-day use bearable. Shows nested folders, threaded conversations and Face/X-Face headers etc. It also has decent keybindings.
The key differentiator for Wanderlust is it's reliable and fast IMAP support.
It also supports a wide range of other protocols:
NNTP
POP(POP3/APOP)
MH
Maildir
From mihai.bazon.net:
I'm currently using it with BBDB (for keeping my address book), Flyspell (spell checking as you type), Mailcrypt (digital signatures and encryption, here's my public key btw).
Can be integrated with Bogofilter, SpamAssassin, and probably whatever you want for spam filtering.
From mihai.bazon.net:
Because Elisp is not a multithreaded language, it kind of freezes while checking for new mail. This can be bad if you're using the same Emacs instance for other purposes, like, writing code. :-) Not the case for me—I don't mind starting a new Emacs instance especially for WL; until one week ago I was using Thunderbird, which needs tons of RAM. Emacs is a lot lighter.
Yep, really. From Outlook-mode, where Outlook-typical TOFU mails are “repaired”, up to a high-sophisticated scoring and SPAM filtering, footer lines with quotes, rules depending on the recipient, MIME formatting, boxquotes, etc.