I think that one of the biggest mistakes that people make when they come to a foreign country to teach English, is that they do not do enough research. Many new teachers sign the first contract that comes their way, without investigating the school, or the terms of the contract. Then they're really surprised when things don't work out the way that they had in mind. I am not suggesting that research and investigation can eliminate every surprise, or that thoroughly looking into the school beforehand will ensure a positive work experience. I'm simply suggestions that a lot of problems and surprises can be avoided by learning as much as you can in advance.
But the school and the contract are not the only things that you should be concerned about. You might also want to look into weather patterns that exist in the area you want to teach. For example, some parts of China get extremely cold in the winter, while other parts are nearly tropical all year round. Some parts of Korea are colder than others. Does it rain a lot in Vietnam? If it does, and you hate the rain, you might want to take that into consideration.
Food is another issue that often goes overlooked. In Korea, for example, you can find just about any kind of western food your heart desires. You may pay a lot more for it than you're used to, however. And you might have to search for a long time in order to find just the thing you're looking for. And what about when your co-workers want to take you out to dinner? Are you prepared to eat what they are eating? What kinds of foods are most popular in Indonesia? If you're planning on coming to Korea, have you ever tried kimchi? You might want to give it a whirl before committing yourself to living in kimchi capitol of the world for 12 months.
Language is another big issue. Everyone you talk to will tell you that you don't have to speak Korean in order to teach English in Korea. And they're telling you the truth. What they're not telling you, however, is that if you live in Korea, then you live in Korea, and you have to figure out how to do everything from ordering food to finding a bathroom. If you can learn a few basic greetings and useful questions before you come, I guarantee that your experience will be much more enjoyable. You can teach English all day long without speaking a word Korea, but when you step outside the walls of your school, you're not in Kansas any more, Toto...
To learn just a few basic phrases that will greatly improve your quality of life, I recommend visiting survivalphrases.com. They have free podcasts and short lessons in many different languages. I used it before I came to Korea, and it worked awesome. I'm now a big fan. There are also lots of resources on youtube, as well as at esloutlet.com (which is updated on a regular basis, by the way).
There are hundreds of things that you might want to research that I haven't mentioned. A few of them might include, but are not limited to: What is the exchange rate? (VERY IMPORTANT QUESTION) or What's the difference between a public school and a private school? or What kind of medical care / insurance is offered? or What is the cost of living? or Will my apartment be furnished or unfurnished? Answering questions like these will help prepare you for the exciting journey that awaits you as a teacher of English abroad!