Building a mail system into a single-threaded text editor seems a bit silly to me...
Why's that? The (not gnus) mail system I use on Emacs will hang the Emacs process for a few seconds sometimes when I hit send while the Emacs process talks to my ISP's mail server, but that is the only way the mail system's single-threadedness ever really bites.
When I check for new mail on my ancient Pentium III system, the Emacs process hangs for less than a second or 2 usually.
Those delays are nothing compared to the time I waste waiting for Firefox to do something.
I prefer the Unix (and Erlang!) model of having lots of independent processes communicating, and individually serving a specific function. While I really like using Emacs's buffer-based environment as the common interface (such as working in a shell buffer and being able to edit results), I'm not convinced that adding more and more functionality to the same process is a good idea from an architectural standpoint. (And unlike Firefox, GNU Emacs doesn't even have good namespacing / isolation for its extensions, hence all of the variables with package-variable-name-with-several-parts names.)
Plan9's ACME preserves most of what I like about Emacs, but it's mouse-chord-based UI is a dealbreaker for me.