Eh, I suppose I should start doing this again when ducks is absent. Bleh... but I'm so lazy. :X
When Shielding Plax comes into play, draw a card.
Enchanted creature cannot be the target of spells or abilities that your opponents control.
If you look at it, it's pretty solid. Without it being a cantrip, it might see some play simply because green has big creatures that won't die unless you target them, which removes the loss of card advantage. But it IS a cantrip, which means it's likely that it WILL give you card advantage, as your opponent has to uberblock your fatty to kill it, unless your opponent is playing green too. And if he does, it's no skin off your nose, as it was a cantrip. Or if you have a key card that you need to protect, like Cytoplast Manipulator, it makes it read "Indestuctible except by Wrath", which defeats the purpose of killing it. I'd say that mainly due to the cantrip effect, it's a solid card if you drop something, anything worth protecting. If not, it prevents the opponent from clearing your creature with a Shock/Helix before their alpha strike. And back we come to the cantrip.
The only complaint I can see about it is that the three mana might be too slow comparitively, as Ohran costs the same amount. But again, if you have a key card that needs to be protected, then this is your best bet. And the "normal" enchant creature drawback is nullified, since, once again, it is a cantrip; so it can only help you if you play it. The only question is if it is worth playing for the three mana, which is probably how you would rate it.