Proper introductions get relationships off on the right foot. "Of all social forms, I find proper introductions to be the most difficult. The combination of unarticulated expectations, conventions, and motives creates great potential for unparalleled awkwardness." ~ Prem Krishnamurthy
USA - Once you're introduced to someone (besides the President and other lofty figures), you can call them by their first name.
Canada- If you don't know someone very well, you shouldn't use their first name, it feels uncomfortable if you do. Public figures are not usually called by their first names, and people who are older than you aren't either. There is sort of a leftover British "elitist" mentality, that is sort of in between Britain and America (like everything else). You probably think you are friendlier than the average American.
Québec (French Canada) - Tutoiement (informal second person singular, accompanied by calling people by their first name) is usually the rule when meeting people of any age, at school and in the business world. However, the elderly, authority figures and other people deserving of more respect get the formal treatment (vouvoiement, second person plural and use of title and last name). You tend to be more familiar and friendly in business than Anglos.
Brazil - Once you're introduced to someone, you can call them by their first name. All public personalities, including the President and other lofty figures, are called by their first names except in an official context.
Colombia - Once you're introduced to someone (besides the President and other lofty figures), you can call them by their first name. You will probably use the first name preceded by a title like "doctor".
Mexico (Urban) - Once you're introduced to someone, you first call them by their university title (licenciado, doctor, ingeniero, arquitecto...), and when your relationship is deeper, you call them by their first name. If you're at high school you often call people by their last names or by their nicknames.
Austria - Unless you're all under about 20-25 years old, or related, or members of the same club, you address people with Sie and their last names when you meet them the first time. Adding their academic titles ("Magister Mayer", "Doktor Novak") is also quite common in formal settings, older people do it all the time. Switching to Du and first names usually takes some time, unless you live in Tyrol, where everyone will think you're from Vienna if you say Sie. A rather silly intermediate variant of addressing people with Sie and their first name (which is comparable to saying "Mr. John" or "Ms. Susan") also exists, but is almost exclusively heard in Kindergarten, dubbed American films and stupid afternoon talkshows.
Finland - You can use a more polite (plural, like French vous) or a more intimate (singular, like French tu) form when addressing people, although the polite form is used only in very formal situations (most people would use it with the president but not with a shop assistant). Politeness is often expressed by avoiding direct reference to person, and first names are seldom used in conversation (often only when you need to catch somebody's attention: "hey YOU there -- listen to me [and shut up]!").
France - Calling somebody by his/her name implies that you know him/her pretty well.
Germany - Unless someone is a child, you have to be very close to him to address him with Du, which usually means that you can call him by his first name. (Else, you address him with Sie). However, students use Du with each other.
Greece - Once you're introduced to someone, you can call them by their first name, but according to social status and the context at hand.
Italy - Once you're introduced to someone, as a general rule you don't call them by their first name and don't use the direct tu pronoun to address them, unless it is a leisure meeting.
Netherlands - With most people you know, you use the informal je form; the honorific u is used in more formal situations, like speaking to older people or to businesspeople from another company. You think people shouldn't make themselves more important than they are, we are all normal people after all.
Poland - You use first names only with friends and family. At work it depends on age difference: you will never use first name when talking to someone much older than you.
Sweden - Once you are introduced to someone (besides the royals and other lofty figures), you can get away with calling them by their first name. You can call almost anyone "thou" in Sweden, if you keep a polite tone of voice, even if you do not know their first name.
England - Once you're introduced to someone (up to and including the Prime Minister), you can nowadays call them by their first name, unless you're in an officers' mess or the Garrick Club.
Scotland - Once you're introduced to someone, you can usually call them by their first name.
AUSTRALIA AND NEW ZEALAND
Australia - Once you're introduced to anyone (perhaps besides the Governor-General, but who is likely to meet him anyway?) you can call them by their first name.
New Zealand - You call everyone by their first name, from the Prime Minister down.
India - Once you're introduced to someone, you will call him/her by name only if the person is a peer, subordinate or junior. Anyone who is senior or higher in the socio-economic ladder must be addressed with great respect.
China - Once you're introduced to people, you can call them by their full name, but depending on their social status, you can call them by their family name plus their title. You can also call them Xiao (young) or Lao (old) plus their family name.
Japan - You almost always call people by their family name, except for your superiors at work and other important people, whom you have to call by their title-- calling your boss Tanaka-san is a big no-no. The only people you can call by their first names are little children, your siblings, cousins, friends from early childhood, and people you meet in bars. You bow to greet people. Once you're past school age, you're expected to know the various methods and degrees of bowing-- from cursory nods to full ninety degrees to groveling on the floor-- and use them properly according to the situation.
You don't know the Emperor's name. Custom dictates that you refer to the reigning emperor simply as tennô hêka (His Majesty the Emperor), and his name (yes, his; only men can inherit the throne) is never mentioned in the media. Deceased emperors are referred to by their okurina, special names given after death.
AFRICA AND THE MIDDLE EAST
Nigeria - You never call a person older than you by their first name. NEVER. It is considered an abomination.
Israel - Once you're introduced to someone (possibly even the Prime Minister, President and other lofty figures), you can call them by their first name. You may even take the liberty of calling them by their nickname - even the Prime Minister, President, and other lofty figures!
Turkey- Once you're introduced to someone you can call them by their first name, but according to social status and the context at hand, you will need to use their title and the proper address form. Last names are never used in conversational contexts.