The bad point is apparently elevating semantics over substance. This time the point is credibility. Central bank credibility strikes me as a fairly unoffensive thing to talk about if what we're interested in involves expectations (which it always does, when we're interested in central banks). When we don't see the Fed moving NGDP, my reaction (and Krugman's reaction) is to say that they have failed to credibly commit to a higher NGDP level. They are doing something, after all. All the QEs and statements that they'll maintain an accomodative policy. They just haven't made a credible case. Under other conditions, that might have done the trick but it wasn't credible. Scott essentially says there's no credibility problem at all, they just haven't done what they need to do.
OK... sounds like semantics to me.
See here's the thing with Sumner and Krugman (who uses the credibility line a lot). Scott will talk ad nauseum about how Keynesians and Krugman are wrong, often because of semantic differences like this. Krugman might not concede - he keeps using his lingo - but he also keeps advocating for good policy rather than investing in someone that fundamentally agrees with him about what the Fed should be doing.
And I keep stressing this because this sort of thing makes good policy less likely, not more likely. So if you're of the market monetarist ilk, take heed and behave accordingly.
The good point is shorter, but still quite good: Scott recognizes that conservative central banking type models depend heavily on what you put in their objective function and the model of inflation that you use. That's a very important point. A lot of people take the traditional version of that model to be gospel truth. It's not, it's just a model.