Anything you write with it now you will be able to play back like this soon. Etherpad is already saving all the keystrokes. They just haven't released the player yet. (This is a one-off prototype.)
http://gvetchedintime.com/gvetchedintime/videos.php
This site shows how some fairly good illustrations are drawn (brushstroke playback). Unfortunately, not all drawings can be animated and the ability to search for playback-capable drawings is limited:
http://www.ratemydrawings.com/
http://www.ratemydrawings.com/drawings/animation/428912.html
It would be interesting to have the timestamp data (if it is recorded) of the keystrokes (adjusted to author's timezone - privacy withstanding) so you can see a clock/calendar [day|night icon] when the author wrote it (was it 3am in the morning? did they put it aside for a couple hours or was it a period of days?).
EDIT: Didn't see this comment. http://news.ycombinator.com/item?id=495812
However, it should be somewhere hidden and available only if people really want to be able to replay. Else it is going to be an over kill on their servers.
What a great way of bringing attention to a start-up: make something that's inherently fascinating due to features. This is unlike anything online I've seen before because the functionality just doesn't exist anywhere else. So it's a great thing to watch and it also tells you about the service.
I like that you keep constantly going back to eliminate unnecessary language. Something tells me that's the first and most obvious trait of watching good writers write: at the governor's school, when the writers would write on paper instead of on the computer, you could read a draft and see that emerging pattern of phrase after phrase, each building on the last. It's interesting seeing it happen on the computer.