Empathy
Why? Because we all have our demons, and being able to recognize that everyone is fighting their own battle (regardless of how trivial it may seem to you) is a formidable skill to have, in my opinion. It allows for understanding, which often paves the way to friendship, rapport, and a multitude of other fundamental dynamics that allow us to foster real and lasting relationships (after all, isn't life all about the relationships we foster?).
It shows character and charisma, and I'm always rather impressed when I come across someone who is empathetic towards others. In a way, it can be a bit surprising (in a good way) to meet someone who seems able to let go of their own problems to try and understand, and maybe even be a part of someone else's, even if just for a minute.
The more you exercise the skill, the more it starts to develop into something that ends up being an important part of your character and persona.
Think about it for a second, and perhaps you'll see that empathy is, more often than not, a foundation to any kind of relationship. Whether it's empathizing with your secretary who has the flu and is a tad late on that report, or whether it's a professor who gives you a couple extra minutes on the test because you got a flat tire whilst cycling to class; it doesn't seem to matter whether it's professional, personal, or sometimes even superficial, a little empathy can go a long way in my experience.
We often forget, but we've all been "there" at one point or another.
Discipline
Life is hard. Some parts are enjoyable, and if you choose to it's possible to make a lot of it fun, but this doesn't lead to success and soon makes one's life feel empty. Genuine happiness relies on hard work, effort and dedication. Being happy with your job means you either have to work hard to be promoted, or network and constantly take up new opportunities. To have a good family you need to first spend years of your life finding the right partner, and then dedicate lots of time to your children. To feel happy intellectually you need to put time into writing books, or doing advanced degrees. Nothing comes easily and all of this takes time and effort.
I went back to university last year, after 7 years away, and was shocked by the difference between the successful students in 2005 and the successful students in 2012. The top students now have been able to not only get a good degree, but also manage a number of internships and research positions, and set up a number of organisations, communities and startups. This kind of effort requires an impressive level of discipline, and the most successful people managed to achieve this at a younger age.
You can train discipline, and if you do so at a very young age, your life will be more successful, easier, and ultimately happier.
I elaborate on this point a bit further, and relate my experience of the difference in the most successful people, in a blog post: The single most unfair advantage a person can get - A Young Entrepreneur in London - by Ivan Mazour.
Assertiveness
Assertiveness, even if you're an introvert, will help you gain respect and save your dignity when you're under verbal attack from an angry or unreasonable person.