The most interesting part is the daily deal (prepaid couponing) as a progression of the CPM>CPC>CPA advertising model.
The opportunity to eliminate marketing/advertising risk as an upfront cost for small businesses is incredibly attractive. Despite the hype, countless copy cats, and bubble comments already, this market is still incredibly young and underdeveloped.
I think the real question is whether this new advertising model is best served by a consumer brand, where Groupon has significant first mover advantage, or an underlying technology solution for every publisher with an audience.
Curious to see if hipmunk expands to other travel products. Tapping GDS systems for inventory should be relatively easy (ex any subscription fees). The challenge will be creating a UI that can correctly convey the value of those additional products.
Airfare really only has a few value variables (price, duration, departure/arrival date, and stops) that are clearly objective. Expanding into other travel products (hotels, packages) and you start adding many more variables and subjective value.
Publishing technology in the 21st century is bringing content creation to the masses.