Brad DeLong does the unpuzzling that, as he notes, shouldn't be needed:
PGL explains what Mankiw should be puzzled about:
Adam Ozimek continues the discussion:
Paul Krugman with more on the topic:
Nothing to add, that pretty much covers it, but I do wonder: When Greg Mankiw asks, relative to 2003, "are circumstances different now?" just how much eggnog has he had?