After launching numerous air strikes against Hamas targets in Gaza, Israel is prepared to launch a ground invasion. Israel has always done amazingly well in conventional warfare against Arab military forces. However, rooting out combatants who hide among civilians has never been something that conventional armies have done exceptionally well. Of course, an occupying army would help cut down on the rocket attacks-they can respond rapidly to launches. However, a conventional army also presents numerous targets for unconventional attacks and an occupation can be a political risk.
One risk is that Hamas can try to drain the resolve of the Israeli people by trying to bleed the ground forces. Losing friends and family in an occupation is never popular and Hamas presumably knows this. With the support of Iran, they can probably wage a fairly effective campaign.
Another risk is that the inevitable civilian deaths can be exploited by Hamas to bolster their support. As they gain local support, they can step up the attacks on Israeli forces and goad them into more retaliatory attacks. This will lead to yet more civilian deaths and thus further enhance the support given to Hamas. Of course, this tactic can be a risk for Hamas. People in Gaza might actually support Hamas less as things become more bloody.
There is also the general concern about how the rest of the world will react. Israel has never been a generally popular state and invading Gaza would generally not be seen as a positive action. Of course, the United States will stick with Israel-they are a critical ally.
From a moral standpoint,the ground war can be seen as morally acceptable in some ways and less so in others. On one hand, ground troops can be more precise in their attacks and can have better local intelligence (relative to air strikes). Of course, ground forces can still make errors and kill the wrong people (civilians). On the other hand, having ground forces invade Gaza escalates the conflict and can very well lead to more deaths. After all, the Hamas targets are not clear military facilities or locations. Rather, their fighters and rocket launchers are spread among the civilians. As such, the civilian population will be rather involved as Israeli ground forces move towards their targets. Unlike aircraft, the troops have to cover the ground relatively slowly and will no doubt be attacked along the way (thus increasing the odds of civilians being killed during battles).
Of course, Israeli cannot stop the rocket attacks simply by continuing to bomb from the air. While some people have argued that air power alone can win conventional wars, history (from WWII forward) shows otherwise. As such, unless Israel and Hamas can reach a political solution, Israel will almost certainly need to invade. And, once again, more people will die on all sides (Israeli, Hamas and civilians).