Most of your work is to learn stuff, to figure out solutions for new problems. However some people keep themselves in their safe-zone, avoiding new kinds of tech or solution. I belief that a good developer will like the feeling of frustration when learning something that they don't know, that a bad piece of code will make then uncomfortable.

The skill that I look for in another developers is the ability to learn stuff that they aren't trained. This way I know that they will be able to solve new problems, to find better solution for old problems. By testing then in something that they don't know, I will see if they are open to learn new stuff and able to apply new concepts, and ask how they felt about it later.