Stay safe & healthy
Yemen is currently at war and under international attack and is heavily damaged. In addition, there have been problems with terrorism and kidnappings of people including foreigners.
Once it is possible to visit Yemen again, the following will again become relevant:
The public consumption of alcohol is punishable under Islamic law in Yemen. Homosexual acts are also prohibited and may be punishable by death.
Driving is on the right. While Yemeni drivers have something of a reputation for bad driving, the reality is slightly more nuanced. Risks are taken, particularly in Sana'a, which would not normally be taken in other places, but the locals expect this to happen and compensate accordingly.
For trips outside Sana'a, however, a 4-wheel-drive is almost mandatory as most roads away from the routes connecting main cities are not paved. Travellers should also give serious consideration to hiring a local driver/guide, as maps tend not to be as useful as they can be in other countries. A city limits border pass is required as only the cities are well protected by the military. It is also worth noting that Yemen has one of the highest populations of armed civilians outside of Texas so be polite.
Tap water should be avoided. To stay safe, it is recommended to stick to the bottled variety.
Additionally, be aware that the country is exceptionally dusty. Travelers with breathing difficulties (such as asthma) may encounter problems in more remote destinations.
The dry air (especially from September 'til April) can be bothersome, causing cracked lips and sometimes nosebleeds. Always carry a Vaseline stick with you, available in most pharmacies in Yemen, and a packet of tissues.
Particularly when hiking, remember that much of the country is at altitude. Therefore, as well as taking the usual steps of drinking plenty of water and protection from the sun (which can be very harsh in Yemen), be aware of any dizziness you may be experiencing due to rapid ascents. Many of the more popular hiking routes are covered in loose stones, so be careful of your footing. Some peak ascents can be at a near 70-80 degree angle, so any fall will be devastating. Be prepared with bandages and/or anti-bacterial creams just in case you get a cut, which is normal during hiking.
Polio and malaria are common to Yemen. Polio is present in some Red Sea coastal towns and malaria is also present in low-lying areas along the Red Sea.