So I've had two questions come up about the shield cantrip, the first is about armor proficiency: does some type of proficiency apply to the use of the Shield cantrip like when using a normal shield? My inclination is no, but I could see either shield proficiency or spell proficiency being applied.
My second question is about when it gets dismissed for 10 minutes. As it's written if it's ever used to block it will be dismissed for 10 minutes, regardless of the damage caused by the hit it blocked. But I'm wondering if the intent might have been if it's used to block an attack that meets or exceeds its hardness then it gets dismissed for 10 minutes. That seems a bit more in line with normal shield use to me.
You don't need any sort of proficiency. You get a +1 circumstance bonus to AC and that's it. The spell ends after the "shield block" reaction is resolved regardless of the result. The bonus to AC is the same as if you were protected by screening, or a parrying weapon, so get to cover or raise your parrying weapon until the spell recharges.
I don't think this is supposed to compare to real shield use, it's more of a temporary HP on a 10 minute timer thing.