My concern with the long sales letter approach is how relevant it is to web apps. Mail order books, courses I can see the benefit, but with a web app, why write lots of spiel about a product someone can start using straight away with a trial account. I am at constant battle with my business partner over this as personally I don't believe it is the right way to create a conversion with web apps (How many successful we…
Look at Basecamp (http://basecamphq.com/). They have a tremendously successful webapp, and their landing page is long enough to clearly explain the benefits and uses many of the features commonly found in direct mail/mail order advertising.