Slurping is a sign of politeness in one country, while in others, it's rude. Eating with your left hand might be a no-no in some regions while in others, no one would ever notice. Traveling and manners come hand in hand, especially when it comes to food and each country has it's own distinct cultural milieu.
In Japan, never stick your chopsticks upright in a bowl of rice. Chopsticks should always be placed together in front of you parallel to the edge of the table. At funerals in Japan, the rice bowl of the deceased are placed with chopsticks upright in the rice.
Never eat tacos with a fork and a knife. Mexicans think eating tacos with anything more than your hands looks tacky and silly, and worse, even snobby.
When eating with your hands in India, don't use your left hand while eating. This is due to the fact that traditionally the left hand is used to wipe after going to the bathroom ... therefore considered dirty.
Italians rarely drink coffee after noon. If you drink coffee after noon, you will branded as a tourist. They never drink coffee after dinner or in cafes later in the day. Interestingly enough though, espresso is fine.
Also, Italians never put Parmesan on pizza like they do over here in the states. A good rule of thumb is if they don't offer the cheese to you, don't ask for it.
In Korea, when an elder offers you a drink, lift your glass with both hands to receive the beverage. It is a sign of respect for elders in Korean culture. After receiving the drink, you should turn your head away and take a discreet sip. Also, wait for elders to start eating until the eldest male has started.
Russians love their vodka and they drink it neat without ice. Adding anything is seen as contaminating the alcohol's purity. The only thing they normally mix with vodka is beer, which creates the drink called yorsh. When you are offered a drink, it is seen as a sign of friendship and trust so it's a good idea to say yes.