I've heard the argument about text width many times and have an impression that it's blindly translated from the printed world without considering the reality of computer screen.
One counter-argument is that the screen is small compared to the size of document. You are peeking at the text by sliding a small hole up and down. This is why many users maximize the browser window. Seeing a bigger chunk of the text at the time helps them to read and scan it faster more than having optimal line width.
Another counter-argument is that you can't control the width well and will probably screw the user. If it's left uncontrolled, one can always resize his rowser window and read at the width one prefers, and this is indeed default behaviour of the Web.
Now making some small margins is good. The old forum used <blockquote> around posts.
Auto guess of format won't work in all cases, as Ben said. Let's leave it implicit.
To say in another words, these two improvements you propose would probably be "too smart" features. I agree with your other two points.
