In geopolitics and history, truth is a matter of perception, which is itself subject to biases. So what happens when those making judgments, the international community, are pulling for one side in a conflict? Objectivity flies out the window, and in its place is an artificial bar being set for perception of victory.
In the conflict with Israel in the summer of 2006, Hezbollah lost 50% of its fighting men. There was billions of dollars in damage to Lebanese infrastructure, and over a thousand dead civilians. Two years later, Hezbollah is going to great lengths to assure Israel that it does not intend to start another fight - an odd thing for a victor to do. And yet, Hezbollah "won" the war against Israel in 2006. The bar of victory set by the international community was merely: survival. Hezbollah did accomplish as much.
Fair enough. However, just because the rules of the game are being changed does not mean the players will no longer want to win. Israel and Hamas have insisted on fighting, and each will attempt to reach the criteria that have been arbitrarily set as being required for victory. Hamas needs to survive, and survival depends on deterring Israeli attack. Thus Hamas will hide among civilians as much as it can, hoping to avoid Israeli firepower. We are seeing a great many civilian casualties as a result. Israel on the other hand needs to completely and systematically crush the entire network of Hamas to be recognized as victorious by the international community. Accomplishing such a lofty objective would necessarily require a ground invasion, and would inevitably result in a lot of casualties among innocents.
We in the international community have changed the rules in this conflict; we cannot be surprised to see the combatants playing by them. The Israeli onslaught has been so exceptionally vicious (or "disproportionate") because we've set the bar of victory so high that anything less would inevitably result in an Israeli defeat.
P.S. This has nothing to do with whether or not Israel *should* be doing what it is doing. A military response to provocation by Hamas may be morally justified, but that doesn't necessarily mean it is wise. The Soviet Union 'provoked' the US during the Cold War (and vice versa). Pakistani militants certainly have provoked India lately, but should India think lightly of attacking an already unstable nuclear-armed country of 180 million people? Of course not. The question we need to be asking is whether or not this offensive is good for Israel's long-term security objectives. Unless there is something big that the Israelis know that I don't, I think the answer will prove to be no. Although, my opinion may be biased by the fact that I think Ehud Olmert is stupid and incompetent.