The difference:
Conquering iraq was easy, and American politicians like easy things. Iraq could be hit overland from Kuwait or Turkey. It could be hit by sea from the Red Sea, the Mediterrenean Sea, and the Persian Gulf. Iraq had a small, decrepit conventional army. Iraq's leader had name recognition, and so the war was easier to sell.
Iran is hard to conquer. It has mountains, a superior military, and can only be accessed from the Persian Gulf. Its leaders, while brutal sponsors of terroism, are not well known. For those more cynical types, you should know that Iran also has lots of oil, and it has the second largest reserve of natural gas in the world.
Iraq was easy.
Iran would be hard.
That is the difference.
Yeah there's a difference there true. Although, I doubt the Iranians would be able to defend their huge coastline, while the Iraq-Kuwait border was a very short stretch of land. Just from looking at the maps, it truly looks easier to attack Iran, if your only base of operations are the Persian Gulf and Kuwait. And before the Iraq war the media was fretting about the 72,000 "Republican guard" and 20,000 "Special Republican guard".
But yes, Iraq was easier, and it was easier by more than most realized. The Iraq regime was a gang of mobsters running the country. Iran on the other hand claims that its brand of religious fundamentalism is ordained by God. No doubt some Iranians actually believe that.
Nevertheless, Iran is a big problem. It is more dangerous than Iraq was. So is North Korea, and what we don't know about what goes on in Pakistan and Saudi Arabia. And being in Iraq really hasn't helped us with that.