1. Mosques need head covers, arms, and legs to be covered. Common areas such as malls and markets are OK with knee-length bottoms and sleeveless tops. Wearing shorts and short skirts, low-cut tops, or sheer clothing is not appropriate. In contrast, wearing loose pants is OK.
Bars and clubs are OK with whatever you wish to wear (call to check the dress code, though). Beaches (public) are few, so you have to wear respectful swimwear. Take a hotel with a beach, so if you desire to wear a bikini, you can do so without any hesitation.
2. Avoid publicly and openly discuss religious matters. There is a risk that your words will be interpreted as a disgrace to the Prophet or the Islam, and - if so - you'll be punished (fine or prison).
3. Tourists are allowed to take pictures, but you must ask permission before taking photos of locals, especially citizens and covered women, hijabi women, women in an abaya. It is forbidden to take pictures of ports, military objects, government offices, mosques, palaces, private residences, industrial facilities, etc., or in other places if marked so with signs.
4. Controversial discussions with police, immigration officers, security personnel, directors, or similar official people are risky, especially if they are Qataris. There is a simple rule that the Qataris are always right. Any of your attempts to contradict them will fail, if not deepening the problem. If you come into such a situation as a visitor or tourist, at least make sure to discuss it in private, because you have more chance to solve your problem.
5. Don’t argue with angry drivers. By the way, the police, the authorities always will side with the Qatari citizens.
6. It is forbidden to say or write anything negative about the ruling family.
7. Do not go in public when drunk and cannot control your posture, body language, and words.
8. If you happen to be an atheist, do not say this in Qatar.
9. Especially in the hot months and Ramadan, the Muslims are active from the evening hours, even until the morning. That is the time when they can eat, drink, and all that in lesser heat. Tourists shouldn't be surprised or complained about these activities, e.g., in the hotels.
10. When you go to public beaches, inform about the dress code regulations, which exist mainly for women. These rules should be strictly observed.
11. Drinking alcohol is restricted to specific locations or areas, such as in assigned bars or hotels.
12. Never stare too noticeably at Qatari citizens, no matter how fascinating or interesting is their look or clothing. That is an essential rule of conduct in Qatari society. If you do otherwise, you will probably risk angry eyes.
Foreign workers make up the vast majority of the population, and they are not subject to such rules.
13. Pointing the finger directly at Qataris should be avoided at all costs. That is not appropriate in your country either, but the negative effect in Qatar is much stronger.
14. If you are a man, avoid talking to an Arab woman in public (e.g., asking for directions). In contrast, speaking to an Arab woman by a female visitor is not a problem. If you are traveling with your family, the woman should talk to the Arab woman.
15. It is an insult to a Muslim man if his offer to shake hands is rejected. However, a Muslim woman can leave shaking hands with you, but that is simply a religious ban for Muslims and non-Muslims alike.
16. Being late to the agreed time in Qatar do not count as rudeness. It is entirely useless to blame a local for not arriving at the exact time.
17. When sitting opposite, avoid pointing the feet, or the feet' soles, as this posture can be considered an insult in the local culture. It is also considered an insult to sit with your back to the person you are speaking to.