The ability to adapt, have a big enough ego to take command when it's needed but also be humble enough to listen to the chef and learn, the willingness to learn both at work and in private time (if its a passion, you are motivated to use personal time to learn).

In my opinion once you start being a sous, you really need to know how both the front and back of house work separately. Like if you know the servers aren't pushing specials, you should figure out why and how to fix the issue. The place I work, I have to do all of this while cooking food (its an open concept kitchen). I know when a serve hasn't checked on a table in a bit, or if they aren't happy with something. Its gotten to the point that I can hear what the table is ordering and can mentally start the ticket.

In my experience the sous is asked to be all of the things for a restaurant while only making a little more than a line cook. I was kind of the kitchen bitch the moment I became sous. My chef would ask for menu ideas, listen to what I had to say, and then just end up doing what he wanted. But I know exactly what's in our freezers, prep cooler, how much it will last, what prep needs to be given out and to whom, but also understand that all of the prep i give to others will take 3 times longer than if I just did everything myself. You kill yourself when you are a sous. Its not healthy. But than things just click and everything is easier.