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…
The argument isn't really about "long copy".
What you need to do is display all the benefits your product offers, show how YOUR web apps are different, exploit your USP and tell the world about it... and debunk all objections.
That could take 500 words... it could take 2,000 words... or it could take 10,000 words.
It all depends. Long copy isn't long because people "want" it to be long. It's long because it takes that long to tell people how awesome the product is, as well as get in touch with them emotionally.
Jeremy