I suspect it was a combination of the words about the immorality of the spending and the overall honest of the speaker.
Obama called it immoral in 2008. Of course after examining his past most of us recognized a lie when we heard it.
And that is true of Romney... politically he is a bit up in the air... but he instincts by which he lives his life are rock solid conservative. So when he says the spending is immoral because of the implications on the future of his grandchildren... that matters. That rings true in a way mere words could never do.