I would think those questions should be asked every time on a case-by-case basis.
But I see your point - historically, projections of military force have happened if it has been in the interests of the country in question to do so. Enemy of my enemy is my ally, but in this case Gaddafi is more than just one autocratic leader of a backward country - he represents all the other dictators of similarly backwater countries.
Certainly Gaddafi has no friends in the West. But to take sides against him and take a position that strongly supports the rebels with military force would set a precedent for all the rest of the countries experiencing civil unrest and uprisings.
It clearly tells the dictators that we're not going to sit on the sidelines and watch you commit the greatest misuse of authority a person can be guilty of and murder their own citizens.
It clearly tells the citizens of those countries that they have our support, that they can oust their leader and if they start killing them, they'll get military support from the west for their cause (either as open military action, equipment drops, or covert operations).
And, you know, in this sort of situation I could condone an intervention perfectly well (with an UN mandate) - if it were just one country in question where the citizens finally got fed up of their Great Leader and stood up for their rights. But it's not. It's pretty much the entire Mediterranean coastline of Africa and most of Middle-East.
What I'm asking is: Can the west (or if an UN mandate is considered, rest of the world) afford to send this sort of message? Can we back it up if either the citizens in all those countries (and this is the worst case scenario) start an uprising against their esteemed leaders, expecting support from us, and said leaders decide to call instead of folding and respond violently?
Do we have sufficient resources to actually deliver sufficient support to the people who counted on it? Or will they be left alone, harbouring a bit more resentment against the West for helping the Libyan rebels but not them?
I don't think this is a situation that has notable precedent in the history. US and other entities supporting revolutionary actions over the world have usually been more or less isolated cases. This is a major unrest/uprising affecting lots of countries simultaneously, and in a way intervening in one of the countries gives a precedent for the rest of the countries.
With this in mind I can fully well understand why every major player seems to be very very hesitant to commit their direct support for either side, outside of political and economical pressure on Gaddafi.
Clearly, everyone's sympathies are on the Rebels' side here, but committing support for them is just much more complicated than it should be.
There are some interesting parallels here to Winter War, actually. The sympathies of practically the whole world were on Finland's side, yet none of them were in a position to give any concrete support. The analogy is far from complete, but basically it's the same effect.
On the topic of UN - I doubt that avenue will result in any sort of military actions unless Russia and China stop their filibustering, but even a humanitarian mission would be better than nothing.
Of course, humanitarian aid would be given for both Loyalist and Rebel occupied areas equally...