Was it ever R&D's intention to allow Karn, the Great Creator to destroy lands in conjuntion with Liquimetal Coating, and if not, why won't you errata that ability to not be able to target lands controlled by your opponents?

Cards can interact with each other in powerful ways. That’s what happens in a game with 25,000+ pieces. We don’t tend to use errata to stop those interactions.

I just listened to your SOM DTW #3, and I'm curious where you draw the line between cards like Liquimetal Coating (which you said allowed Red to deal with enchantments too easily) and cards like Lux Cannon. You seem to have great enthusiasm for the latter, and hold the former in disdain, but they all seem to be similar to me.

I am okay with artifacts doing some things the colors cannot as long as it requires some actual investment. Lux Cannon has to be cast and then four turns later you are able to use it. Liquid Metal Coating is too quick and easy to circumvent red’s weakness.