There is only one way to kill a career: Stop writing.
That's from Dean Wesley Smith's latest blog post in his Killing the Sacred Cows of Publishing series. If anyone is timid about letting go of their manuscript and sending it out into the world of editors and agents, I would recommend reading this article.
... if you go into everything you do in publishing believing the myth that you can make a mistake and kill your career, you will make all your decisions from a position of fear. And you will make horrid decisions.