Postal Mail: Anyone can mail you advertising. Members of the Direct Mail Association voluntarily honor a national list of people who don't want it. (This list was probably set up to help avoid the imposition of legal restrictions.) The cost imposed on the sender are fairly high: The typical fancy direct mail advertising piece costs around a $1 when you add up all the costs. The cost imposted on the recipient are less and it is not particularly intrusive. "Automated" junk mail is legal.
FAX: Junk FAXes are illegal. They would be pretty cheap to send. While not exactly intrusive, they cost the typical recipient in supplies, blocking the line for a time, etc., and are more of a pain to sort out than junk mail.
Phone: Automatic junk phone calls are illegal. A human can call you but they are restricted by time of day, etc., and must not call you back if you ask to be put on their no call list. This is probably the most intrusive form and is thus the most tightly regulated. It would be pretty cheap to do by machine, but that is now illegal.
Email: I would say this is about as intrustive as a FAX. It uses up recipient resources about as much. With good receiver technology it might be reduced to the intrusiveness of postal mail. But the key thing is that it's incredibly cheap to send automated email. The cost is reportedly about 0.06 cents per message in bulk, which is how spammers can make money only charging 0.1 cents a message. It's this chepapness that really upsets the balance, being about 200 times cheaper than postal mail. In a recently New York Times article a marketer was quated as saying it makes no different if you anger 100,000 people by sending 1,000,000 pieces of mail when 10 of your targets buying your product means you make a profit. It is this sort of thinking we are fighting. Unfortunately, the law hasn't caught up with email yet and the application of other exsting laws is questionable.
Since spammers can use throw away accounts and host addresses, in some cases there is nothing you can do on the network to hurt them after the spam. For this reason and because so many voters will be annoyed by spam, I think criminal laws against it are inevitable. But this list isn't the right place for any legal discussion. There is plenty that can and needs to be done technically.