There's a meme circulating among those that opposed the war that we don't really know how bad things would have been and that there's no way of knowing. In a facile sense, this is true. We can't know the future. However, there have been a lot of civil wars in the second half of the 20th century. Here's what a few of them look like. (Numbers are for the losing side and wars posted in no particular order because I'm lazy. All of these conflicts are primarily civil wars, though some do involved intervention of outside powers, more on that at the end of the post.)
Algerian War: 800,000+ civilians killed or wounded
Hungarian Revolution of 1956: 2,500 killed, 13,000 wounded, 13,000 imprisoned, 200,000 refugees estimated 350 executions
North Yemen Civil War: 100,000 dead
Zanzibar Revolution: 2,000 - 4,000 with estimates up to 20,000 civilians killed in aftermath
Indonesian Invasion of East Timor: 60,000 to 100,000 soldiers and civilians
Internal Conflict in Burma: 121,000 killed
Greek Civil War: This one's a mess and representative of other political conflicts that lasted many years and with multiple sides. The winning side claimed about 16,000 dead all told and the losers just under 40,000. Another 1,000,000 were displaced. Other major civil wars of this complexity, such as Congo's, have been omitted.
Al-Anfal Campaign: Saddam Hussein had a much more effective army than Libya. This is what he did to combat an uprising: 182,000 civilians killed. I think this is probably the most comparable of these as to what could have been expected in Libya if Qaddafi had his way. In these sorts of conflicts, the worst killing happens after the main fighting is over.
Nepalese Civil War: 12,700+ deaths
1994 civil war in Yemen: 7,000 - 10,000 dead, appearing twice on the list is one reason why I don't think we're going to get involved in Yemen. In this case most casualties are listed as soldiers, but I question how distinct the civilian/soldier divide is in a weakly institutionalized state such as Yemen.
Spanish Civil War: 500,000 dead on both sides. Both sides engaged in substantial killing of civilians
This is just a brief round-up of what I could easily find on Wikipedia. While there are rational reason to oppose intervention, it does look like the kind of terrible outcomes of many of these conflicts have been avoided. I find it unlikely that the rebels will feel able to act without outside support, while there will very likely be reprisal killings by the rebels afterward I expect these to be at the very low end for a civil war. If Qaddafi had instead hung on to power, I would have expected towards the higher end since he would have already been angry at the other powers and seen little reason to take their reactions into account. It's simply not true that we don't have any idea what would have happened if Qaddafi had succeeded, reprisals after a civil war are a nearly constant feature of them. While there are exceptions, such as our own Civil War where post-war executions and extra-judicial killings were fairly rare, these are very rare exceptions and there is absolutely no reason to believe Qaddafi would have acted as the exception, rather than the rule.
Of course, there are a couple of complicating factors. First, most 20th century civil wars have involved an element of great power competition. This surely lengthened them, raised the stakes, and thus increased the casualties. Second, many of these civil wars involved some degree of outside interference, which is exactly what we're doing in Libya. While this gives some reason for doubt, the unusual combination of substantial defected forces and a huge disparity in arms quality between those intervening and Qaddafi's forces gives a strong reason to expect a different outcome. More detail could certainly be given for these differences. But, at a glance, there's good reason to believe that the situation prior to intervention would have looked much like most civil wars, while the ways that this intervention differs from earlier cases gives good reasons to believe that casualties will be relatively small.
That said, I won't be at all surprised if the scale of what happens approaches Srebrenica (8,000) in total, though not in any one place and including battlefield deaths. This will still be ugly, it's against the mind-numbing horror of what most modern civil wars end up looking like that intervention is justified. The sheer scale of destruction caused by civil wars is rarely appreciated when so few of them occur anywhere near reliable reporters and television cameras. As long as there is some form of international force deployed in whatever settlement arises from this conflict, I believe the worst of the reprisals will be avoided.