Web browsers don't word-wrap lines in submitted text, which makes editing a page that someone wrote in a web browser annoying (gqip is vim user's friend here). Is there any way to improve this?

See "using the web interface with a real text editor" on the tips page. --JoshTriplett

Would it be useful to allow a "max width" plugin, which would force on commit the split of long lines ?

Please, no. That would wreak havoc on code blocks and arguments to preprocessor directives, and it would make bulleted lists and quoted blocks look bogus (because the subsequent lines would not match), among other problems. On the other hand, if you want to propose a piece of client-side JavaScript that looks at the active selection in a text area and word-wraps it, and have a plugin that adds a "Word-Wrap Selection" button to the editor, that seems fine. --JoshTriplett