Learn how to excel at your class basically. We had a guy in our old raiding guild back during Vanilla who raided as balance the whole way. He absolutely loved the class and spec. It was the only character he ever played. He had zero alts.
I remember him quite well because he was a good guy and I was an altoholic of sorts. I remember wondering how someone could only ever have one character. But he was good at playing his balance druid and was a benefit to our raid team. Any class or spec can be viable if played in the right hands.