The plastic filter thread. For a lens that costs $2300 and has an L on it, the filter thread must be made of metal. Kudos to Ken Rockwell for pointing this out.
Your loss, I guess. The Mk II is a phenomenal lens.
There is a practical reason why Canon made this change. Plastic does not gall to metal filter rings, unlike metal filter rings. How many people have experienced stuck filters?
I prefer metal thread too, but I don't have any problem with the plastic thread in my 100L.
Still, $2,300 is kinda too much. If it had IS, maybe I would have been an early adopter. Since it doesn't, I'll wait until I can find one for around $1,700 or so.
I could probably live with the plastic threads, but I will never purchase this lens for anything close to $2300 no matter how good it is, and I shoot for a living.