Maybe you're not that organized of a person and would rather just throw some junk in a suitcase than make a packing list and pack everything on it (*cough* me), but if you do that with Iceland, then you'll end up having to buy a few things while you're there (like gloves or deodorant, perchance?). Packing lists are just a smart idea, especially in cold places where you'll be for an extended period of time.
So, disregarding the normal packing list that consists of "pants, shirts, underwear, toothbrush" (I'm hoping you can handle that part), here's my Iceland packing list with the things you'll need but maybe didn't think of!
Layers. I'm talking short sleeves, long sleeves, light sweaters, heavy sweaters, light jacket, heavy jacket, jacket or sweater with a hood, warm socks, etc... The weather in Iceland can change drastically throughout the day so you want to be prepared for its crazy-intense winds in the morning as well as its Michigan-springtime weather in the afternoon and then the snow at night.
Gloves, hat, and scarf. Depending on the time of year you go, these are valuable accessories. Not only is Iceland cold, but the winds are strong, so when it would otherwise be a fine temperature, the wind makes it chilling. You'll want to be warm.
European outlet adapter. All the plugs in Iceland are going to be different from the ones in the U.S.A., so you'll want to be sure you have an adapter in order to charge your devices. (Here's a link to the ones I got off of Amazon, but you can find them in most Walmarts. I just like the one I got because it was a 2 in 1.)
Chapstick. If your lips are like mine and tend to get chapped while traveling, you'll want chapstick for this trip, no exceptions. If you don't tend to get chapped lips, you still might want to bring some chapstick along because of the high winds and cold—it's just a recipe for chapped lips, and why buy that in Iceland?
Spending money. Like I've maybe mentioned before, everything in Iceland is expensive—2 to 3 times more expensive than the States. So if you plan on bringing back any souvenirs, eating out a lot, or hitting the bars, be sure you have some extra dough set aside for the trip.
Wipes. Okay, so, bathrooms in Iceland can be scarce to come across, and when you do come across them, most of them you have to pay to use (they're called water closets in Iceland, and most of them are marked with a "WC"). This in no way means that you'll have to go to the bathroom without a toilet, but when you're in the middle of a 5-mile hike and there's been no bathroom for the last four hours, you just might just have to use nature as your bathroom (you also might not, but bring wipes just in case!).
A water bottle. Water isn't free there—even many restaurants charge you if you want a glass of water—so it's a smart idea to bring along a water bottle and fill it when the water is free (like at your hotel room). Beware that the water often smells like sulfur (or rotten boiled eggs), but it's cleaner than any water you'll find at home. Drink it. You'll be fine.
Travel toothpaste, deodorant, shampoo, etc. In case you didn't know, the airport won't let you bring your 15.6-liter shampoo bottle on the plane. You'll have to buy travel sized everything that's even somewhat liquid. This is common sense, but some people forget about it. So don't forget!
Some kind of navigation. Check out my post next week about driving in Iceland for more details!
Good hiking boots (preferably waterproof). There's so much walking to be done in Iceland if you truly want to explore it, and a lot of that walking is around waterfalls or snow-covered mountains (or melting-snow covered mountains). You'll want your feet to be comfortable and dry.
A decent camera. I don't think I need to explain why.
In the end, just remember where you're going: a cold place that's more expensive than home with a lot of breathtaking things to see. Be sure you prepare yourself to see it!