Maybe this should be enchant creature. It doesn't make very much sense to have it target either enchantments or other (non-creature) artifacts. Also, I can't remember if they closed that loophole with Planeswalkers and their counters (I seem to recall that they did) but if they didn't it might be broken with Planeswalkers activated abilities.

It's possible that by enchanting enchantments or lands you'd get some interaction you normally don't (through artifact matters or destruction effects). Also, I'm not sure what loophole you're talking about.

In one of MaRo's recent podcasts he said one of the things he would change about Liquid Metal Coating was that it could make Enchantments into Artifacts allowing red to destroy them which is out of color for red.

Considering we've been following R&D processes to date I thought we should conform to what they would do in our place.

As for the loophole, there used to be a combo with Experiment Kraj where if you got a +1/+1 counter on a PW, you could activate the abilities using Kraj. I wasn't sure if we might create a similar situation if a PW was turned into an artifact and ceased to be a PW.

In any case I seem to recall them closing that loophole.

Liquid Metal Coating is colorless, this is blue. Personally, I think letting red kill enchantments using a double blue card is fine.

On the flip side, you could say blue shouldn't be killing enchantments either, but I also think blue killing enchantments with a red splash is fine. To me, this is in a similar vein to Turn // Burn; combining two colors do to something neither can do alone.

March of the Machines + Liquimetal Coating + Quicksilver Elemental (or Experiment Kraj with a counter on a planeswalker) = Quicksilver Elemental with planeswalker abilities. Spam ad nauseum.

Yes, this was closed in a recent rules update that makes loyalty abilities only able to be activated by planeswalkers.

And only once per turn, regardless of permanent type.

We can't simple make it "Enchant creature." This way it would fall off the instant it hits the battlefield, as the permanent it enchants would not be a creature anymore. Putting the whole Animate Dead change of "Enchant Creature Card in a Graveyard" to "Enchant Creature" on it is too far from clear design.

Speaking from a flavour perspective, it's awkward for this to be able to target enchantments. Cast Agoraphobia in steel? Sounds like modern art ;) Maybe this issue is neglectable, as the design is far more pure the way it stands.

It wouldn't be the first card in modern magic to be able to change an enchantment into an artifact, Liquid Metal Coating for example does something similar albeit temporary.

Considering Theros I think this isn't too bad.

Any type of permanent can activate loyalty abilities. This does nothing to shut down planeswalkers except to make them vulnerable to artifact removal. Note that it does nothing against enchantments, either, and doesn't stop any abilities of the enchanted permanent.
I don't think the reminder text on this is right, either. Or perhaps it's more that I think it should be explicitly stated as rules text, because what happens when you remove the "land" type from something with subtypes is confusing. I think this needs to say "Enchanted permanent loses all types and subtypes and is an artifact," or something to that effect.
In some ways this is very strange, because it's a blue Pacifism/quasi-Stone Rain (against basic lands).

Link is correct. I misremembered the update. This card does almost nothing to combat planeswalkers. It makes them a little better in that they can no longer be attacked, but also a little worse in that they're susceptible to artifact removal.

I would say:

Enchant creature or planeswalker
Enchanted permanent is an artifact in addition to its other types.
Enchanted permanent's activated abilities can't be activated. If enchanted permanent is a creature it can't attack or block.

Wording taken from Faith's Fetters. Yes it makes it more wordy, but I think the flavor is more important than having a short line of text in this case.

I think you misunderstand. If you make a creature an artifact (just an artifact since it loses all of its other types) it wouldn't be able to attack or block as its not a creature anymore. That line of text (Can't attack or block) isn't needed.

Other than opening other permanent types up to artifact removal, it's effectively identical to Pacifism.

I just realized that you could cast Cast in Steel on another Cast in Steel to have it fall off a creature but remain on the field as a artifact. You could have an aura enchanting an artifact that used to be an aura enchanting an artifact. Lol, I love this card.

...And now I have the mental image of the aura literally falling off and making a clonking noise as it hits the ground. XD

Perhaps the most interesting thing about this is that it does indeed protect planeswalkers from attack. I do think it needs to make clear that subtypes are also lost.

Is "loses all other types" not clear?