Living abroad teaches us so many things, but the most important are those learned about ourselves. Some would be good to know ahead of time, while others can only be learned through experience.
1. Shyness is a luxury. As an expat, you HAVE to get used to meeting and socialising with strangers...unless you are ok with being very lonely. As I've said before, to begin with, accept any invitation you receive in your new country, and go from there. You'll have loads of choice if you just reach out.
2. On the same note as above, being an expat is an excellent opportunity to develop interests and talents you've always had but perhaps never put into action. This 1) lets you create something of your own in order to gain independence and 2) keeps you busy and meeting new people. Being an expat is an excellent confidence builder if you treat it as an opportunity.
3. Being an Expat Partner/Spouse adds a lot more complexity to a relationship, making essential relationship values such as flexibility and patience even more important. Both partners need to be committed to being open minded about the challenges they will face when one or both are moving to a new country. New expats need to be a bit more open to trying new things, and those partners who are already in their home country need to accept that flexibility is essential and some things they take for granted may not be (at least immediately) acceptable to their immigrant wife/husband/partner. Stubbornness and resistance to change can cause roadblocks and unhappiness in any relationship, but can be the downfall of an expat relationship. If you can handle a relationship abroad with grace and success, you can handle anything!
4. Culture shock is not something that may happen, but something that will happen. For example, English speaking peoples do not all share the same culture! The differences in cultural values and norms can be even more confusing because our expectations don't always allow for this. The two most shocking cultural differences for me have been in relation to 1) customer service and 2) class-ism. Customer service in the UK, NZ and Australia is just not the same. Thankfully, Australian customer service is better than the UK and NZ, but it still doesn't measure up to the standards I am used to. And in regards to class, the English are over the top in regards to class-ism and even 'take the mickey' (or poke fun of) themselves on this one. Australians are in direct opposition to this. God forbid you mention that something is too working class in Oz (such as my neighborhood), you'll instantly be branded a snob.
5. Being an expat is something you don't recover from, in my opinion. I'm sure there is a better way to say this, but once you become an expat, you are always an expat. It is true, in a sense, that 'you can't go home again'. Don't underestimate the profound change becoming an expat will have on your life and you as an individual. It is work, and it is challenging, but overall, it's fun!