One factor that hbr is not mentioning is business owners (national and local) that participate on Groupon, for the most part, are having bad experiences. Obviously this depends on the industry (sky dive plus all around, coffee shops down all around) but not all industries can live off of 25% of their goods sold. I know many local business owners who ran a Groupon and promised themselves that they will never Groupon again because of the poor customer experience, and the large financial loss per Groupon.
The scale part is what worries me. I don't think Amazon needed as much money as Groupon needed to scale which is the biggest worry. At some point, the financial well will dry up on Groupon.
When that happens, I don't know what will become of Groupon. Maybe they should have taken the purported Google deal of a buyout for $6 billion :).