The difference between the situation in Lybia and Iraq.
1. The reasons for invading Iraq were that British and U.S. intelligence claimed without a shred of evidence that Saddam Hussein had weapons of mass destruction. As it turns out, he didn't. In Lybia the stated reasons are that Khadafi is terrorising his people who were allready in Uprising against him. Unlike the reasons for the war in Iraq, this is factual information!
2. By taking away the strong leader in Iraq the underlying tensions between the Sunni, Shi'ite and the Kurds resulted in a bloody power struggle. In Lybia there already is an alternative and widely suported authority in the form of the rebellions.
3. Following point two one can assume that the situation in Lybia will be mostly resolved after Khadafi is releaved of power. Unlike Iraq what turned out to be a years long mission for the U.S. costing them billions of dollars.
4. The UN security council actually suports the actions in Lybia, unlike Bush's invasion of Iraq which was strongly condemned by most of the world.
5. The "U.S." are not the (only) ones taking initiative in this one. The Arabic Liga are actually the ones who requested the UN security council to institute a no-fly zone and most of the (military and diplomatic) work is done by the French, British, Italian, German, Canadian, Danish, Belgian, Greece and even Turkish armies.
6. There arent any U.S. ground forces in Lybia, which means that there is less chance of U.S. lifes lost.
If you are going to compare Lybia to any situation in Iraq I would equate it to the first gulf-war rather then the second, in that this is a situation where the entire world more or less unanimously decides that immediate action should be taken to prevent direct innocent bloodshed. Although I myself would sooner compare it to the war in Yuguslavia in the mid 90s.