The risk factors section (starting on page 11) is particularly enlightening.

I agree, and these two would concern me if I were considering investing:

-Retain our existing merchants and have them offer additional deals through our marketplace;

-React to challenges from existing and new competitors.

Groupon could be facing the double-whammy of existing merchants in many markets having no incentive to offer follow-on deals while threats from the competition would erode margins. These are threats to existing revenue streams, not just to revenue growth.

In my view, investors in Groupon must be betting that they can successfully translate their current traction into a more sustainable business model (i.e. Groupon Now). At this valuation, not a bet I would take.

Pretty standard from what I've seen, actually.