Okay, let's say I buy the argument that there is a lot of money to be made becoming the premier "real-time" intermediary between consumers and long-tail small businesses.

I'm not entirely clear on why Groupon is uniquely positioned to become this intermediary. It seems like there are a lot of other national companies already doing business with this long tail (e.g., eBay, Yelp -- not to mention MasterCard, Amex, the big telcos, etc.)

What am I missing? What about Groupon's relationship with these vendors gives it a leg up?

I don't think it's its relationship with vendors so much as its relationship with consumers. People are flocking to Groupon looking for places to spend their money.