I don't like that week numbers are (documented as) mandatory. I wouldn't see myself grepping on a week number. I'd end up constantly looking up the correct week number in order to annotate each event properly, with no benefit in return. Then again, could write a macro/plugin thingy for one's editor of choice, to fill in the week number for you (ditto for day of week, which I would at least find quite useful, but which would similarly be a right pain to have to fill in manually all the time).

Also: claims to be "one thing per line", yet allows and encourages multiple events on one line (as long as they're on the same day). This is a calendar - is an event not the main "thing" we're dealing with?!