Does the Germanic Consonant Shift illustrate linguistic change in pronunciation, grammar, vocabulary, or spelling?
At first (of course) it was a shift in pronunciation.
Then it was expressed in the spelling.
If we compare Dutch to German, we see that a lot of the consonant shifts didn't happen, like the p->pf or the k->ch.
Dutch ROOK, German RAUCH (ignore the vowel-shift)
Dutch KOP, German KOPF.
Understanding the consonant shifts, and the umlaut (vowel shift) makes German a lot easier to understand (if you're Dutch).