One can also usually distinguish juniors and seniors by the level of sophistication of their code. It gives us a good idea of which candidates will be able to work to our standards, or learn to quickly.
However, that's only one component of success. We find that personality is far more important (though also much harder to interview for).
We need people who are capable of self-management, independent work, and structured thinking in an unstructured environment; who relate their work to the priorities of the rest of the company; who communicate clearly and proactively; and who are collaborative and supportive of one another.
People with all those attributes are remarkably difficult to find.
I love developing it was the best career move i've ever made, but you know what? I also love friends, hiking, jogging, going to the gym, video games, playing guitar, and watching movies.
I don't live to work. I work to pay for the stuff I like to do outside of work.