A common myth about language is that some languages are more difficult to learn than others because they are more "complex". Let me reassure you that so far, no linguist has managed to prove that any language is more complex than any other.
There is a common misconception among some people, and non-linguists who claim that they are linguists (translators, language teachers and polyglots) who claim that linguist Noam Chomsky classified languages by order of difficulty. While Chomsky did classify languages by grammar typology, his classification was never intended to show that any language was more difficult. Some people do claim that Chomsky classified English as easy, Korean as medium and Russian as difficult, but such people never actually read Chomsky's typology.
Is Korean a difficult language? Every language is equally complex, and past the age of 12 every language is difficult to learn. This for two reasons:
1- the part that processes language in the brain is fully developed around puberty
2- Human beings need to understand 98% of the vocabulary used in a conversation or book in order to understand it.
Yes, if you're reading a book or listening to someone talk and that you don't understand 98 of every 100 words he says, you will not understand his message. This implies that when you read that book or take that class in your native language and that you don't understand what it's trying to say, you may feel stupid because you think you should understand it since it's your native language. But the reason you can't understand a book or a class, even in your native language, is because you're not understanding that 98% words that you should know to understand it.
For speakers of Chinese, Japanese, Vietnamese or any language that borrowed a lot of Chinese words, Korean may be an easier language to learn and understand because though they may not know all the words that are being used in the conversation or book, they can infer its meaning because the word in their native language is likely to sound similar. For speakers of languages which did not borrow a lot of words from Chinese, understanding Korean can be a bit of a challenge.
Korean teachers often encourage their students to watch the news, listen to the radio and read the Korean press, but many foreigners will be nowhere near understanding 98% of the words used in such contexts. Experiments on children revealed that children learn how to use a word after hearing it 14 times in different contexts and sentences. So indeed reading or listening to anything in Korean will teach foreigners Korean long term, meaning that if you watch the news everyday you may end up being familiar with the vocabulary which is being used. But that is only related to the understanding part.
Speaking Korean involves two things: the raw language and the "pragmatics" or culture and etiquette surrounding the language. While most foreigners form sentences with minor grammatical mistakes, errors in body language, intonation, and the rules of conversation may not get the message across. These rules are very different from society to society as it is even difficult for Americans of a certain society to adapt with Americans belonging to a different subgroup.
Finally, Korean, just like any other language, is spoken and understood the same way as it is learned. If focus is put on conversation with Koreans, that aspect of the language will be mastered, if focus is put on reading, the foreigner will be fluent in reading but perhaps not in conversation, and people practicing sets of grammar exercises will be good at solving grammar problems but not at actually speaking and understanding the language.