"Our business has grown rapidly as merchants and consumers have increasingly used our marketplace," Groupon wrote in the S-1. "However, this is a new market which we only created in late 2008 and which has operated at a substantial scale for only a limited period of time. Given the limited history, it is difficult to predict whether this market will continue to grow or whether it can be maintained. We expect that the market will evolve in ways which may be difficult to predict."
Other risk factors for Groupon include: failure to retain or acquire subscribers; failure to stave off net loses due to operating expenses; failure to add or maintain new merchants; failure to hold off competition and clones; failure to recover subscriber acquisition costs (marketing expenses such as the infamous Super Bowl ads); disruption of email chain or email restrictions; international growth problems.
Those are the highlights of Groupon's risks, though the S-1 does go on for about 10 more pages outlining potential harmful market conditions including government regulation of e-commerce or losing members of the managerial team.