First, of course, was 9/11 and the subsequent lumping of Iran by the Bush administration into the "axis of evil" category. (For some in the Bush administration this labeling of Iran as "evil" appeared to be justified as the post-Iraq war insurgency began to flare up and it was soon discovered that Iran, if not outright directing much of the sectarian violence by providing weapons and such, was at least encouraging much of it.) The second major development was the "election" of Ahmadenijad as the Iranian president who before long began to make many in the West, and especially in Israel, uneasy with his vitriolic speeches that among other things repeatedly denied the Holocaust and called for Israel to be "wiped off the map." And lastly recent intelligence estimates, particularly after 2007, seemed to indicate that Iran was closer to nuclear capability than previously thought. All of these factors combined to renew the IC's effort to bring pressure on Iran to forgo its nuclear ambitions. Surprisingly, a number of sanctions have been passed including a European oil embargo set to go into effect in June. (Unfortunately, it is likely that many European countries will find ever ingenious and elaborate ways to circumvent the embargo as they did with the Iraqi "Food for Oil" program in the 1990's.)
Obviously, the major concern by the IC is that the Israeli's are getting ever closer to preemptively striking Iranian nuclear sites. That this is highly probable is undeniable since the Israeli's have thus twice before acted (Iraq, 1981; Syria, 2007). Rightly, then, the world is concerned about the repercussions that would surely follow from an Israeli airstrike on Iranian nuclear facilities.
Now for my own part, from the viewpoint of US strategic interests, permitting Iran nuclear weapon capability is probably not as problematic for the region as some suggest and would not directly affect the national security of this country. Therefore, I'm inclined to view the Iranian government as a "rational" actor on the international stage that wouldn't recklessly use its nuclear weapons. Of course this is not what many fear would happen if Iran had nuclear weapons. The more likely scenario is that Iran might smuggle nuclear weapons to one of their proxies such as Hezbollah or Hamas to use against Israel so that the IC could never officially link any use of nuclear weapons against Israel to Iran. Still, I think it is unlikely Iran would act in such a fashion.
But having said that I want to stress that it is important not to be glib about the security threat a nuclear Iran poses from the viewpoint of Israel. Israeli security calculations have to take this situation seriously because it has been in a position since 1948 of actual and virtual war which combined with the history of Jewish persecution that tragically culminated in the Holocaust has, quite understandably, developed into a national security consciousness with a strong existential flavor. Thus, asking the Israeli's to explain exactly how and why a nuclear Iran would be dangerous for the region is an irrelevant and meaningless question to put to them.
Also while many in the West have no problem shrugging off Ahmadenijad's speeches (specifically the ones that have called for "Israel to be wiped off the map") as mere rhetorical flourishes the Israeli's cannot afford to do this. That this is simply political grandstanding cannot (and should not) even factor into Israeli security calculations.
Usually, I'm cautious about using historical analogies since they are often misused, but if you remember Hitler gave a speech in 1938 which called for the "destruction of international Jewry". At that time, like Ahmadenijad's diatribes today, the speech was waved off as typical antisemitic blustering, exaggerated political sophistry. Then seven years later the world learned that Hitler had "succeeded" in destroying 1/3 of world Jewry.
So then the greater challenge isn't getting Iran to give up its pursuit of nuclear weapons but rather of getting the Israeli's to modulate their own security interests to coincide with those of the United States and the rest of the IC. But this will never happen. Rightly so Israel will act out of what it perceives is best for its own security, regardless of what the rest of the world believes it should do. As would the United States, as would any country for that matter. And therefore whether we disagree with the Israeli position or not concerning its fears of a nuclear Iran it is vitally important that we seek to understand that for Israel this is indeed an existential threat.
In short, no matter how vain or empty threats made towards the Israeli state by a country such as Iran might look to the IC, it is impossible for Israel NOT to take each and everyone of these seriously. Its own history demands such.
Thus, we need to be careful when discussing Israel's security concerns vis a vis Iran that we do not do so in a cavalier manner as, tragically, I've seen too many political commentators do.