Castilian is regarded in many parts of the Spanish-speaking world as the standard for proper speech, and the RAE is viewed as the ultimate lexical authority. Most Spanish spoken outside of northern and central Spain does not employ the Castilian lisp or "ceceo" and other features of the Castilian accent. "Ceceo" is itself a shibboleth, as it contains two lisped "c" within the word itself. Castilians do engage in one simplification that is not followed elsewhere, called "loísmo" - the substitution for the direct object pronoun for the indirect object pronoun, "le." The correct usage for "I gave her a smile" would be "le di una sonrisa." The loismo would be to say instead "la di una sonrisa." The RAE has not accepted this practice since the 18th century. The matter is a bit murkier because "sonreír" means in some contexts "to smile at" and not just "to smile." So, "I smiled at her" (taking out the "give" part) would be "le sonreí a ella" (adding the clarification of the pronoun at the end of the phrase. Technically, "her" is the direct object of the verb in this sentence. But "le" is preferred nonetheless. One can think of it as the work of the personal "a" or perhaps just a recognition that some verbs naturally take a dative object.
Voseo. Voseo is the name of using the second person plural form of "you" in Spanish. English and all Romance languages all have three forms of address for second person plural. Typically, they are the "tu" form, the second person singular, which historically has been reserved mainly for family and intimate friends; the "vos" form, the second person plural, which has historically been reserved for friendly acquaintances and as a respectful form of address for one's family elders; and the "Usted" form, the third person singular, pressed into service as a "polite" and neutral way of referring to a person without saying "you." In English, it would be the "Your Grace" or "Your Majesty" form of address. In Spanish, Usted is a contraction of "Vuestra Merced," or "Your Mercy." In Portuguese, the equivalent term is Você, which comes from the same origin. Most speakers of English are unaware that the "tú" form (which is "thou, thee, thine") has faded, and that we are all using the second person plural - the intermediate formality - for everybody.
In Spain, the "tú" form is still used in the proper context, as is the "vosotros" or "you" form, and the polite "usted" form. In many parts of the Spanish-speaking world outside of Spain, the "vosotros" form has atrophied. But in certain regions the "vos" pronoun has remained, usually without the second person plural conjugation. The use of it is called "voseo." Countries where "vos" rather than "tú" is employed include the Southern Cone (Paraguay, Uruguay, Argentina and Chile) as well as parts of Western South America (regions of Ecuador, Colombia and Venezuela). The "vos" form is also quite common in Pacific Central America from the Mexican state of Chiapas, to Guatemala, Honduras, Nicaragua, Costa Rica and El Salvador. And just as the use of "tú" will vary widely from quite liberal, even for strangers (as in Mexico) to quite conservative and only for the family and good friends (as in Colombia), so also will the use of "vos" have a wide range of levels of informality.
Ustedes. Spain still employs the form of "vosotros" or "vosotras" for the plural "you." In most of Latin America and elsewhere, this form seems a bit old fashioned, though it is still understood. It is the form spoken in the most widely-distributed versions of the Bible in Spanish. Therefore, outside of Spain, there is no distinction between a formal "you all" and a familiar one - it is all the same: "ustedes."
Word Selection. Those who enjoy the differences between American English and British English will often cite curious options for word selection. For example, "suspenders" hold up pants in the States and stockings in the UK. In the UK a "pecker" is the upper lip. In the US it is not. In the same way, many New World usages have not been adopted in Spain. "Mantequilla" means "butter" in Latin America, but it is "manteca" in Spain. "Manteca" in Latin America means "lard." Certain words used in common speech in Spain -- coger and concha, for example - have come to have only very rude meanings in many parts of Latin America (in this case, to have sex and the female pudendum). "Pinche" in Mexico is not considered a polite adjective for anything, and means an intense and lowbrow version of "lousy, measly, rotten and damned." It's a hair pin in the Caribbean. "Coche" and "Cochecito" in Mexico and elsewhere mean stroller. In Spain it means a car or a "dear little car." "Cochino" on the other hand, means pig-like and dirty in Mexico and Guatemala.