I speak Mexican Spanish and find it challenging to understand certain accents particularly from El Salvador and Argentina. And I never bothered to learn the 2nd person plural form of verbs and pronouns (i.e. "vosotros") because it is rarely heard in Mexico. But it is used extensively in other countries; I heard it used quite a bit in Chile and Argentina.
Spanish slang is fascinating and rich, especially when you get into the creative use of curse words and vulgar slang which often varies regionally. For example I had no idea that I'd get laughed at if I said to a waiter or shopkeeper, "Quisiera una papaya bien rica" in Cuba which, of course, is simply "I would like a nice tasting papaya (fruit)" in Mexico. And in Mexico the various grammatical forms of "pinche" (literally "kitchen boy" or "kitchen assistant" but in Mexico is the equivalent of the English "f-word") are legendary.