Brass really gets them going the most, but they also hate nickel silver. Just not enough brass in knives for them to really get their spleens going.
I think they hate anything that will tarnish. Recently one knifemaker was criticized for having an exposed carbon steel tang 'cause it could get some mild rust and maybe a few small pits with sweaty palms...So why's that bad? On vintage Western knives that is called character.
Materials that change over time with use and the elements have been part of artistic design and construction forever. There is a certain fetish among knife lovers to never have a piece change. At the opposite spectrum of collecting is a friend who collects antique fireworks. Once a year he and his buddies go out and destroy part of their collections for fun by setting them off.

My neighbor sold me his music CD for $5, it comes with a strikenaywhere match attached to the sleeve. You're supposed to listen to it, transfer the music whatever media you like and then burn the CD with the supplied match.