Off the top of my head, I'd say the top 10-15% of restaurants would have no interest in trying Groupon because it cheapens their brand. Then account for chains who don't really need Groupon's reach to offer such promotions (or rather will find it more cost effective to do something like this themselves).
As for the rest of the restaurants, once the novelty of groupon wears off, how often will they come back to give the general public 50% (actually close to 75% off, because Groupon takes half the cut) off meals ?
The Groupon Now idea is a bit more interesting but I think it's something that's going to be best captured by a less physical method of scaling (i.e. not emplying 4000 sales people)
Disclaimer: I'm currently working for a startup in the restaurant space though it does not have any element(s) of a daily deals/groupon clone.