Barring the obvious things, like family, friends, etc, I've compiled a short list of the things I miss the most about the United States. I have found these things to be lacking pretty much everywhere I've traveled outside of the United States (excluding Canada, which is basically the same as the US. Don't say that to them though, because they really hate it), and what's more, they are things that can't really be shipped, so you truly do have to be in the US to enjoy them.
Things I miss:
Cold, plain, real, non-UHT (that's the milk that is boiled and sealed so it can last for three months unrefrigerated on the shelf, cow's milk.
2. Real Showers
Two words: Water pressure! Actually those words could also be "hot water", or "clean water", or "consistent water", or any number of different pairs of words to describe what I have found lacking in other countries when I've taken showers. I still have a scar on my foot from where electricity exited my body from touching an electric shower head in Brazil. In the war zone you almost always have to walk a distance to get to the showers from your living space, which if the temperature is below freezing and you are still wet from the shower, it can be a very uncomfortable walk. It's also hard to bring all of your clothes to the shower as well, so you usually are in just your shorts, flip-flops, and a T-shirt.
3. Real Toilets
The US is the only country that seems to take their toilets seriously. I ask a question of you other countries: "What the...?" In Brazil you can't flush toilet paper in the toilet, so they have a trash can next to the toilet to put the paper in. In Afghanistan we have "the shelf". Don't ask.
Bare feet. No worries. Enough said.
5. Consistent power
Power out here is also consistent. Consistently unreliable.
7. Real Milk
Worth mentioning twice.
They have grass out here but they usually roll it up and light it on fire to inhale the smoke.
9. Washer/Dryer in your house
I know that even a lot of Americans don't have this, but the difference is, they CAN.
10. Drinkable water
You can only bottled water as an American in most other countries. I can't wait, no more getting all the way to the bathroom and realize that you forgot your bottled water for brushing your teeth.
There's a lot of cool things to enjoy when you go to another country, but there are some things that are so nice to come home to!