b.) Mindshare as barrier to entry -- I don't buy the mindshare argument when the a strength, as noted in the article is the urgency, the deal (the store is the brand that matters -- probably as much as the conduit if not more). Beyond that, mindshare is primiarly money invested into viral loop on facebook/myspace and brand ads.
c.) Infrastructure? Um, it's just lead generation -- I see the larger infrastructure as outward bound direct sales to merchants. (sites like leapfish.com have sustainable conversion rates on outward bound telephone sales to merchants -- online are they important ? well...)
There are already huge investors knocking off the idea, and as for the cash issue, the article talks about the "negative working capital" needed.
However, I think one of the reasons for their success is due to their roots - The Point, the startup using collective power to help people deal with local issues. Groupon's founder once said, "None of us would work here if it were only about selling stuff at a discount—ultimately, we're trying to give people an excuse to get out of the house and enjoy life."
I think a successful company needs to have a corporate mission that is bigger than making a profit. I believe their roots help them build a strong brand and user base, which is a very strong barrier to entry.