Health and Safety
Laos's health care is nowhere near to Thailand's. If you will be traveling extensively, consider buying international health insurance that covers evacuation to Thailand.
Take the same health precautions in Laos that you would in Thailand. Pharmacies are stocked with Thai antibiotics and often staffed with assistants who speak some English. Vientiane is malaria-free, but if you're visiting remote regions, consider taking prophylactics. HIV is widespread in border areas. Reliable Thai condoms are available in Laos.
Laos is fairly free of crime in tourist areas. Pickpocketing is rare, but you should still be careful in crowded areas. Never leave luggage unattended.
Penalties for drug possession are severe. Prostitution is illegal, and $500 fines can be levied against foreigners for having sexual relations with Lao citizens (how this is enforced is unclear, but even public displays of affection may be seen as shady behavior).
In the countryside, trekkers should watch out for unexploded ordnance left over from the Vietnam War, especially in Xieng Khuang and Hua Phan provinces, and in southern Laos. Don't wander off well-traveled trails. Better yet, trek with a qualified guide. Do not photograph anything that may have military significance, like airports or military installations.