I was going to say the same thing. I probably edit at least 50 percent of my comments here and on reddit within a minute, and delete about 25 percent. Presumably a pause in mail sending would have a similar effect.
Maybe we'll see this ... obviously we all thought of this feature before and would enjoy this. Maybe they are listening?
Also, if they are listening I would like to be able to Twitter within Gmail. Have an inbox tab and a Twitter tab; spend all my time in Gmail & refresh Twitter a few times a day, why not consolidate?