Anything you can do on the envelope to impart urgency is worthwhile. For that reason, many mailings may start by asking for action with an indication that you might miss something if you don't open the envelope. As he rightly surmised, this had precisely the opposite effect. People couldn't resist opening. Sometimes the type of postal indicia used can lift response.
Attractive ones seem to lift response slightly.
It's difficult to predict what will and what won't work. If, however, you wish your mailing to have a quasi-official appearance, then a manila envelope may be the best approach. What is more, human beings are very strange. One client a few years ago tested an envelope which required the customer to buy their own stamp against his usual reply-paid envelope. Believe it or not, the reply-paid did not win this test.