The ability to use google. Really.
Not that I would never help anyone, specially a junior colleague. But there is nothing more frustrating that having someone not doing due diligence in solving problems, continuously.
My advice to junior soft. engineers is to do your due diligence in solving problems. Use google, wikipedia or stackoverflow diligently and methodically when you don't know something.
Don't wait too long before going to a more senior co-worker for help - that depends on the type of organization, though. In a good company where people help each other, if you spend more than a half a day trying to find an answer, don't hesitate to stop your own research to ask for help.
In other organizations where people are always mean to each other, you might have to spend a few days on your own (documenting everything you have done and everywhere you have researched) before asking for help. In such companies, documenting what you did helps so that you can show them that "you did google it" because that will be the first retort they'll throw at you.
But regardless, the essence of what I would look for in a junior engineer is to do his due diligence in trying to solve a problem instead of expecting to have answers handled to him all the time. In a nutshell, show me you can use google.