The definition of a transaction is one party giving something of value to another party for something of value. In most cases, the exchange is even. You give a guy ten bucks for ten bucks worth of oranges or tomatoes. An email address is the same thing as accepting money from somebody. If you don't believe me, then check some past court cases. Many judges have ruled this to be the case.
Having said this, you should think of the people on your mailing list as customers, whether they have given you money or not. They have provided you with something of value and you should do the same in return.
Put some thought into what you send out as your enticement offer. After all, people are PAYING YOU for it! Also, this is the product that they will judge you on. You only have one shot at that first impression.
When you are accepting money, you make it a point to offer your customer value for his money. Don't you think that the same frame of mind should be used when accepting an email address?