Depends on the type of writing I think. Personally I would say the most important things are passion, writing about what you know, being genuine and being prepared to do lots of research.
Obviously basic language skills are good, but you don't need to be William Shakespeare. As long as you can get your point across and you have something good to say you'll be great :)
A teacher I had once told me when I did a particularly bad essay; "If you listen to most people's conversations, they're not actually saying anything. There is no point to what they are saying. Make every word have meaning."