Apparently Israel is against international justice. If the PA is recognized, its citizens are subject to the ICC, so they could be prosecuted for crimes against humanity. On the otherhand, Israel is not a signatory to the ICC nor a member, so the ICC has no jurisdiction over Israeli citizens (at least that is my understanding).
ICC jurisdiction applies if either the perpetrator is a citizen in ICC member state, or if the crimes in question occur in a member state. So if Palestine manages to become a member, it would mean Israeli leaders could in principle be investigated and prosecuted for war crimes in West Bank and Gaza.
Of course, since Israel has not ratified the treaty, it would not have to extradite the accused. Also, it would open up prosecution to Hamas leaders for war crimes as well. I think it's unlikely that Israel would actually get into any trouble, so far ICC has mostly been used against poor African countries, but it would be an interesting development and definitely an improvement in international justice system... but only if Palestine's acceptance is strictly by the book. There is a risk that ICC's reputation might be tarnished as single-issue anti-Israeli insitution like the UN human rights commission.