"Reality is painful — it’s so much easier to keep doing stuff you know you’re good at or else to pick something so hard there’s no point at which it’s obvious you’re failing — but it’s impossible to get better without confronting it."
It is a measure of a good technology hire. I don't necessarily want someone who is an expert in one language, one technology or even one particular vertical. I want someone who consistently pushes to learn more. I would even provide the necessary tools/training and time for them to do so.
It is a problem with a lot of developers. They get into a comfort zone and stick it out.
Good interview question:
"Tell me about a time in which you were trying something new and it didn't exactly work out as you expected. What did you do next?"