Being in business school, I hear it all the time about the power of the first impression. It's safe to say people who use the service using a Groupon/Living Social deal are much more likely to not receive the "full experience" because the workers/owners are tired and stressed. THEREFORE, the first impression isn't going to be great. THUS, the Yelp review is going to reflect that. It's important to keep up with the demand and keep that great impression as well maintaining reputable status. Once the reputable-ness is gone...it's hard to gain it back.

Why a business shouldn't do Groupon/Living Social deals. 

As for the pizza guy, if you can't measure how successful the promo deal was...it was pretty much useless.