I wouldn't call genesis-mining a scam, but it IS a clever way to make money from the mathematically challenged. The classic mining contract "scam" works as follows:

1. Say at current difficulty, each unit of mining power would net you $50
2. Sell 12 months of mining power at a price of $300
3. Let the user come to the conclusion that they are getting $50 x 12 = $600 worth of mining at $300. What a great DEAL
4. Neglect to mention that mining difficulty increases and price may wildly fluctuate, and that the 12 months of mining power is likely to be worth only $150 at the end
5. Profit