NFZ is a tactic. Very doable, relatively low-risk although not risk-free, but still a tactic. To what purpose? What further expectations will this raise?

As far as the strategic message to the extremists, what happens if the NFZ doesn't change anything on the ground? Does this make the US look like a "weak horse" once again? Conversely, even if effective, might it support the Al Qaeda narrative of "Crusaders killing Muslims to steal their oil"?

As to using a NFZ as an excuse for wider ground attack missions, what would be the second and third order effects? Just because a lot of Libyans want to get rid of Qaddafi by force doesn't mean they want someone else bombing their country. The historical record is that such interventions are more likely to unify the population under the current regime than cause them to rise up against it.

Overthrowing Qaddafi might be a desirable objective, but then what? Why do we think the near-term outcome of deposing Qaddafi in Libya would be more favorable to US interests than at least the first 5 years after we got rid of Saddam turned out to be?

If Qaddafi is defeated, with or without external intervention, the odds of political chaos and a humanitarian disaster ensuing are pretty high. Further slaughter as the winners take revenge and/or continue the fighting to decide who amongst them will replace Qadaffi is also rather likely.

Merely deposing Qaddafi isn't going to make Libya suddenly look like Switzerland (or even Jordan, to use a less sarcastic metaphor).

The Colin Powell warning about "You break it, you buy it" is well worth keeping in mind here.

"Just do SOMETHING" rarely turns out to be good policy.