Most importantly, spam will inspire serious anger and hatred from a large percentage of the people you send it to. Since spamming is against the rules of nearly all ISP's and web servers, your admin will receive complaints and they will delete your accounts. People running these mass mail programs often approach businesses with little Internet knowledge and promise them huge responses. The results are usually more along the lines of losing web and ISP accounts as well as dealing with obscene phone calls and email from angry spam recipients. Sometimes these opportunities pop up unexpectedly so think of a good way to work it into a spontaneous conversation ahead of time.
Many profitable sites offer something interesting or useful at no charge.
They are able to sell products or services almost as a byproduct because the users are there for the free content and happen to see something they want to buy. People are not stupid, they can tell the difference between a well written site that provides something useful and one that was thrown together in a day just to sell a product or provide a place for banners. Respect your users and give them credit for being smart surfers, insult their intelligence and you will never see them again. If you think about it, many opportunities crop up in daily life to pass on your URL to people. A business card with your URL printed on it will come in handy for this purpose, don't just rely on memory to bring them to your site.