(PS: we have been trying to do such a split test for Visual Website Optimizer homepage. Will post results once we do it)
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.
There's a pretty big difference from Basecamp's website vs someone trying to get you to buy "Forbidden neck pain reliever that the pharmaceutical companies don't want you to know about!!" or a cheap saw made in china that will probably break after a month and cost too much to send back for the Money Back Guarantee*.
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
How does a long sales letter form compare to an approach of "Get the user using the product as quickly as possible", with a few bold benefits outlined, a huge "Try Now" button and not commitment to buy. Of course this is something that I should A/B test myself to really find the answer.