It doesn't seem to me like Groupon's getting any special pass for being 'popular'. When I search "my city" deals, I see about a dozen other sites on Adwords doing the same thing or similar. Most I've never heard of.
works for the coupon sites but if you tried to implement the same method for a strictly content site, you'd be in big trouble.
Had the feeling for a while now that coupon sites can get away with more than other kinds of sites. Organic and PPC both.