One important aspect of English-language education that often gets overlooked is, I believe, pronunciation. I have met many students of English who are, for the most part, quite fluent, even able to get through a master's program in an English-speaking university, but who constantly struggle to make themselves understood to native-English speakers. This is true not only of students, but of professors in universities, as well. Not only is it frustrating for both speaker and listener, but, it may also make the non-native speakers question their language abilities and doubt themselves.
I think the problem is easily solved, but it requires explicit instruction. The vast majority of students are not able to pick up the correct pronunciation by simply listening and emulating a native speaker. In addition, in all languages, there are specific aspects of an ESL student's first language that cause difficulty when learning English, and these must be acknowledged and understood.
Here are a few common pronunciation problems for Korean speakers when they are learning English, because I'm too lazy to write them all out myself.
One of the most important differences between English and many languages is that it is a stress-timed language, as opposed to a syllable-timed language (such as French and Japanese). Syllable-timed languages give equal weight to each syllable in a word, whereas English (and German, Russian, and Mandarin) are stressed-timed languages, in which certain syllables and certain content words (such as nouns, verbs, adjectives and adverbs) must be stressed. Alongside the stress, of course, must be taught the unstress (the schwa - the most common sound in the English language). English teachers need to focus on word and sentence stress/unstress patterns (which I am not going to define or describe further, since voluminous amounts of information exist already on the topic, and there is no need for me to repeat it here). Just know that as a teacher, you need to be aware of it, and of the addition interference or difficulties that your students may have based on their first language.
Teaching American English Pronunciation, by Peter Avery and Susan Erlich, is a book I would not teach without.
And here are some techniques for teaching pronunciation.
Here's yet another good site to get you started.