My Inner Cynic says: "great idea!" might be a way of trying to get rid of people in the easiest way possible, since it doesn't cost anything? Maybe not, but if you have nothing to lose because you're just some random guy who wants to disengage as quickly as possible, it might be easier than arguing about why it's a bad idea, especially since you don't really care.
Yeah, that's exactly my point. While I don't necessarily think they were trying to get rid of me, I don't think they really understood the pitch. So that's one of the signs to look for - if they just say, "That's great, good job!", you're probably not engaging them. If they grill you a little bit further, then they're at least slightly interested in the idea, and they've understood the majority of it.
One point of the starbucks pitch is quantity over quality. You may get a lot of people who will kindly blow you off, but you're not paying for any of it, so you can continually practice until you can engage any of them with an interesting pitch.
