Giszmo, great questions.

We did a blog post on this as well:
http://blog.coinbase.com/post/64143333723/building-a-world-class-remote-customer-support-team

Writing skills would be important, but mental acuity is more valuable to us. You can be located anywhere in the world. Also, timing is not as important as correctness on the test. Hope that explains a bit and thanks for applying!