Taboos and pitfalls
A practical list, with the caveat that Beirut, Cairo, and Riyadh are not the same place.
Generalizations about "the Arab world" are usually wrong. Twenty-two countries with different histories, demographics, and norms cannot be reduced to one set of rules. Beirut is not Riyadh; Tunis is not Sanaa; Cairo is not Doha. What follows is a working list of habits that travel well across many contexts, with notes on where they vary.
The body
The left hand. In many traditional contexts, food is eaten with the right hand and items (especially food, papers, gifts) are passed with the right. The left is associated with personal hygiene. In Beirut or Cairo professional settings nobody will care; at a Bedouin meal it matters. The safe habit is to default to the right hand for handing things over, eating shared food, and shaking hands.
Shoe soles. Showing the soles of your shoes — by crossing your legs in a way that points the sole at someone — reads as disrespectful in many contexts. In Levantine and Egyptian formal settings it is mildly rude; in Gulf majlis settings, more so. Sit with feet flat on the floor or with crossed ankles.
Shoes indoors. Most homes are shoes-off. Watch what the host does. In some Levantine and Gulf homes, slip-on slippers are offered to guests.
Family and marriage
In conservative Gulf and tribal contexts, asking after a man's wife by name is sometimes uncomfortable. The polite move is to ask after "the family" or "Umm [oldest child's name]" rather than the wife's name. In Beirut, Cairo, Tunis, Amman, and most cosmopolitan settings this is not an issue; first-name use for a man's wife is fine if you know her.
Asking why someone is not married, or has no children, is intrusive in any culture and especially so here. Do not ask. If they raise it, you can engage; if they do not, do not.
Religion
For an outsider, religious topics are best engaged with curiosity and restraint. Asking respectful questions ("how is Ramadan going for you?") is welcome. Critiquing Islam in casual conversation, joking about religious practices, or offering theological corrections is not. Christian Arabs face a different version of the same dynamic; do not assume that an Arabic-speaking person is Muslim, and do not be surprised if a Christian Arab uses Islamic-origin religious expressions (see religious expressions).
The relationship between "Arab" and "Muslim" is complicated. Most Arabs are Muslim, but tens of millions of Arabs are Christian (especially in Egypt, Lebanon, Syria, Palestine, and Iraq). Most Muslims are not Arab — Indonesia, Pakistan, Bangladesh, and India together have many more Muslims than the entire Arab world. Treating "Arab" and "Muslim" as synonyms is a common American error and reads badly to people who have spent their lives navigating exactly that conflation.
Pork and alcohol
In Muslim contexts, pork is essentially absent and alcohol is restricted to varying degrees. Saudi Arabia bans alcohol entirely; the UAE and Qatar regulate it tightly; Lebanon, Egypt, Morocco, Tunisia, Jordan, and Iraq permit it with regional variation in social acceptance. In many homes you can be served alcohol; in many others you cannot.
Do not bring alcohol as a host gift unless you know the host drinks. Sweets, fruit, or flowers are universally acceptable. If you are at a Muslim host's home and there is no alcohol, do not joke about it or ask. If you are in a Christian Lebanese home and arak is on the table, the social rules are different.
For Muslim friends who do drink — and many do — do not assume the drinking is hidden because it is shameful. Some treat it as private; others are open about it. Take the lead from them.
Politics
Political topics vary in how openly they can be discussed. Egyptian politics, Saudi politics, and Syrian politics are conversational minefields with people who could lose their jobs or worse for the wrong opinion in the wrong setting. Lebanese politics, by contrast, is discussed continuously, openly, and at high volume.
Israel-Palestine is universally engaged with, but the engagement is not optional, and the assumed positions vary. Do not press; listen. The Iraq war, the 2011 uprisings, and the post-2011 trajectories of Syria, Libya, Yemen, and Egypt are similarly live topics. Palestinian, Syrian, Iraqi, Yemeni, and Lebanese friends may have personal stakes that make the conversation different from a discussion of similar events somewhere else.
Public-private
Public displays of affection vary widely. Hand-holding between same-sex friends (men with men, women with women) is unmarked and friendly in most Arab countries. Romantic affection between men and women in public is more restrained than in most of Europe or North America, particularly outside of cosmopolitan urban areas.
LGBTQ identity is legally and socially restricted across most of the region, with substantial variation. Lebanon and Tunisia are more open than most; the Gulf and Egypt have legal restrictions and active enforcement. This is a genuine safety question for travelers, not a politeness question.
Photography
Do not photograph people without permission, especially women. Do not photograph government buildings, military installations, or officials. In some countries (Egypt, Morocco) tourist-area locals expect a tip for being photographed; agree on this before, not after.
Variation
Re-emphasizing the disclaimer: these are tendencies, not rules. A Beiruti professional, a Cairo university student, a Riyadh businesswoman, and a Marrakech grandmother will hold different views on most of the items above. The most useful skill is reading the specific people and place you are in. Default to slightly more reserved than feels natural to you; relax as you read the room.
The other useful question is whether what you think of as a "regional taboo" is in fact something with no specifically Arab content. Punctuality varies by country. Tipping varies. Bargaining is expected in suuqs and is not in supermarkets. Direct eye contact is normal between peers and may be lowered slightly with elders or in formal settings, similarly to many Mediterranean cultures.
"Arab" vs. "Muslim"
One last note. Use "Arab" for cultural and linguistic context (an Arab restaurant, Arab music). Use "Muslim" for religious context (Muslim-majority country, Muslim holiday). The two overlap heavily but are not the same. A Lebanese Christian is Arab and not Muslim; an Indonesian Muslim is Muslim and not Arab. Getting this distinction right is a basic competence and saves a lot of awkwardness.