The worst quality that a developer can have, is a carefree attitude. When you are assigned with a task, you should have a connection with it and feel motivated to complete it. I have seen people who don't care what the product is about. Only thing that matters to them is moving the clock. That person might be skilled and might have a lot of knowledge, but, if he doesn't care about what he is doing, he cannot enjoy the task, and thus cannot contribute to it in the right way.
The second worst thing is arrogance, and it is true in all fields. With knowledge and understanding comes a feeling of superiority. It's ok to feel good, but that knowledge should not be used to dominate, intimidate, or belittle someone. You are a junk if you do that.
The third thing which I find disturbing is inability to take chances. We all get saturated and bored, and it is natural, but our primitive instinct of security prevents us from taking the decisions. This will suck out the energy from you sooner than you'll realize. Take chances, always go for learning and exploration. Happiness is what matters the most.
Hope I'm in the right light. :)
Brandon
Frontend Developer