I hope this isn't overly semantic, but I'd like to gain some clarification on the use positive (or negative) externality. 

I usually shoot down suggestions for examples of positive externalities like organic agriculture. We can grant that organic agriculture is better than conventional, but that doesn't mean it is producing additional external benefits. Instead, organic agriculture might feature fewer negative externalities compared to the status quo. 

I've even seen examples like smoking being a positive externality because smokers die sooner and save governments money on Social Security (perhaps that accounting might be questionable, but let's grant it for the sake of argument). Again, my semantic preference would be to label this as the absence of status quo negative externality.

If the semantics aren't interesting, might there still be something interesting on the tax vs. subsidize side? Returning to the agriculture example, taxing the negative externalities associated with conventional agriculture seems smarter than subsidizing organic. This seems like a case where it's more efficient to regulate the end and not the means. But what market characteristics might make subsidizing specific not-negative-externality-producing things more efficient?