Amen to this post on intertwingly. Sam Ruby notes that while the SOAP based web services specifications allow for all kinds of things, they don't allow for not doing all these things but keeping it simple instead. They really should. The desire to build something that lasts is a good one, but simple things have to always stay simple.
The standards stack around email is a good example both of why one tries to build something extensible (if you use every tweak of the standard you're sending around some very contorted messages with every possible type of encoding and character escape you can think of) and an example of how to build something that's extensible, but where the extensions don't hurt the simple implementations. If you don't use all kinds of fancy features of email the standards are really, really simple.