This one is necessary for any IETF effort, but I mention it anyway
to keep it visible from the start:
- users demand interoperability. A message should be usable regardless of
the implementation used by the sender or recipient (provided both
implementations are standards-conforming).
Being able to read referenced content in threads without having the
original material copied has already been mentioned. Here is a related
issue:
- users want to be able to see relevant related material in context, in an
appropriate order of presentation, and without having to read
irrelevant parts of previous exchanges. [Yes, that means "top-posting
is abhorrent" (possibly excepting languages where reading bottom-to-top
is normal)]
- users want to be in control of the use of their resources (communications
bandwidth usage, storage, execution of programs, etc.) -- they don't
want automatic execution of executable content from untrusted sources,
for example.