There is some major new stuff here over v1.6.9, including:
HTTP, HTML support (a functional www browser!), including proxy support.
News posting.
Message threading. (like news threading)
Display MIME images inline as well as text/html and enriched text etc.
DSN support.
Unseen-only display window.
multipart/signature support, "Intelligent signatures"
PGP key management can talk directly to the key servers over the web,
eliminating the email based query delays
Address book stuff, aliases etc.