The thing is they have professional graphics, high profile clients, high quality video testimonials & upfront pricing. The website is actually pretty free of gimmickry. While it is long the actual text is only about 3 paragraphs. Also you're on their homepage, which is more trustworthy than your e-mail inbox.
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*.
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.