Good technicians just have that "knack" for it, I don't really know how else to describe it, they have solid basic skills and know how to apply it practically, the really technical stuff is just learned but a good tech can apply it.
A slightly dated example, they know when a customer says there is static on the line to ask, "Does it sound like bacon frying, a old scratched up vinyl record, or is a rhythmic ticking sound" and know how to interpret the answer.
Any "good tech" can learn the whatever basics needed for voice networking, VLANs, applications, system programming, or whatever else, but they need to be well rounded and understand how to apply those things to the real world... Not just "book smart". Basically I am saying they need to well rounded.