This issue is borderline between design and implementation, but the
decision will affect lots of code to be written/changed later, so I'm
posting this here, in case there are some considerations that I
missed.

I've got lots of complaints about the way most bidi-enabled editors handle keyboard and cursor interaction; would a list of specific annoyances, er, use-cases be of any use?

For example, in Mellel (http://www.redlers.com/) I've discovered that if an RTL document starts with an LTR string, it is impossible to simply insert a linefeed in front of that string. It's also a major pain to edit XML in a bidi editor. And I do mean major. If emacs could make it easy and natural to perform such operations it would be manna from heaven.

Cheers,

Gregg