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Do's:
1. Do smile in the face of culture stress.
2. Do wear a head scarf and longer shirts, ladies.
3. Ladies: do wear pants, NOT skirts.
4. Men: Do wear pants and shirts with short sleeves (no shorts or sleeveless/tanks)
5. Do be polite and greet people, say "Salam."

Don't:
1. Don't point the bottom of your feet at someone.
2. Don't drink alcohol in public
3. Don't engage in PDA's (public displays of affection)
4. Don't accept an invitation from an Afghan on the first offer - you need to have received an offer at least 3 times for it to be legitimate and sincere.
5. Don't lose your temper in public
6. Don't shame an Afghan - try to "go

Is it hard to adjust to local custom?

Yes and No - depending upon how adventurous you are and willing to be flexible in how you do things. 

Even clothing will go a great deal - if women are considered modest, the men they are with will get more respect. So show honor and respect in the culture - it is a honor/shame society.

There are a ton more helpful hints at:

Great words of advice I would also add to respect elders. Not to look at women, especially young women.

Any experience of living in a middle eastern and/or muslim country will have direct relevance in Afghanistan.

The expectations and demeanor of the people, when confronted with a non-muslim foreigner can vary greatly depending on your location in the country.

Whilst security is an issue throughout the land, on the whole, the people in Kabul are exceptionally friendly, welcoming and ready to accept you.

Afghanistan is espetacular.

Is not growing a beard going to cause a bias?