It's the fault of movies like the insipid Vicky Chirstina Barcelona, not to mention our dwindling knowledge of the world outside our own backyards, that most people have no idea that the native language of Barcelona is Catalan and not Spanish (Castilian). In fact, most expect to drink sangria, see bullfights and hear flamenco guitar when they come here--all of these, though occasionally available in Barcelona, are products of the radically different culture of the South of Spain. It's the same kind of geographical and cultural dimness (often ascribed, but by no means exclusive, to North America) that leads people to believe that we Canadians have polar bears living in our backyards and skate to school on permanently frozen rivers. We don't, but I have heard that Australians do ride kangaroos to work.
If you know only a few things before you come to Barcelona, know these. Barcelona is the capital of the region of Catalonia (Catalunya around these parts). It's a cosmopolitan city of about two million on the Mediterranean coast of Northern Spain. Before being incorporated into modern Spain, Catalonia was a separate kingdom and a proud nationalism still courses through the veins of most Catalans. While a minority are separatists, contrary to the unfortunate reality of the Basque countries to the north, there's absolutely no messing about with bombs and violence here.
The language, as I've mentioned, is Catalan, but all Barcelonians also speak Castilian Spanish (the Spanish of Latin American and the rest of Spain), often mixing Catalan and Castilian when amongst friends. Tourists can expect a reasonable level of English (and sometimes French or German) in hotels and many restaurants, but, once off the beaten track, may have trouble encountering English speakers. (The last statistics I read placed those Catalans who speak English with some degree of fluency at about 30% of the population.)
While Barcelona has gained a relatively liberal reputation, its citizens, particularly the older ones, are a fairly conservative group and are sometimes scandalized by what the tourists get up to--stumbling half naked and piss drunk through the streets as some are wont to do after their days of baking on the beach. The truth is that the attitude of the locals is still largely "live and let live", but recently, tourist excesses, which haven't let locals live very well, have many Barcelonians thoroughly fed up. I'll write more about that and how you shouldn't behave in Barcelona in another post.
For your convenience, here are a few other tidbits that might be of use:
When to Go: The summers are hot and sticky (25-30 C) and much of the city closes down in August for holidays. The best time to come is spring (April/May) or fall (September/October) when the weather is milder (15-23 C). Winter is quiet, rainy and much cooler (5-10 C).
Getting Around: Barcelona's main tourist areas are within easy walking distance of the centre or a short metro or cab ride away. The metro system is extensive, safe and cheap (77 cents a ride if you buy the 10 trip card). It runs until midnight from Sunday to Thursday, until 2am on Fridays and all night on Saturdays. Taxis are reasonably priced as well and cabbies usually aren't inclined to rip you off. Be prepared for small supplemental charges, however, at night, on holidays and when travelling with luggage or to or from the airport or train station. Finally, the city is very bike friendly. Your hotel can help you with rentals and routes. The red and white "Bicing" bikes so prevalent in the streets are for locals who subscribe to the Bicing service. (See the links to transport sites on the left hand side of this page for more information.)
Tipping: Catalans tend to tip very little or not at all. 5% is generally considered adequate in restaurants. It's not expected that you tip in bars or cabs. If you want to tip, any loose change you might have is usually more than enough.
Be Warned: While Barcelona has relatively little violent crime, pickpockets and purse-snatchers abound, particularly in tourist areas. You don't need to outfit yourself with a money belt and personal alarm system, however; just try not to look like a mark. That means dressing for the city and not like a tourist--you wouldn't walk around your town in little more than a bikini top with a knapsack strapped to your chest, for instance. You should also use reasonable precautions like not leaving bags unattended (I always keep my purse on my lap and never hang it on the back of a chair or leave it sitting on the ground), keeping valuables and money you don't need in the hotel safe and being aware of your surroundings. As there are various scam artists about, you should never (and this should be obvious) hand over your passport to anyone on the street, even if they look like a cop. And, as the Simpsons should have taught you, there is no such thing as a wallet inspector.
On the up side, you might be heartened to know that in my more than two years here I've never been robbed (knock on wood) nor have any of my 40 or so visitors. It's just a matter of being ever so slightly on guard, just like in any city of some size.