The disadvantages are the same as wearing regular armor you lack proficiency with
(Note, I used the basic rules pg.44 as a reference, but the information should be the same as in the PHB, pg. 144)
You claim that shields are not armor. I challenge this assertion. Shields are not body armor (as noted in this answer on the first linked question). The answer on ...
A general preface: each edition of D&D is its own game, and rules should be evaluated as such. Just because things happened in old rule sets does not mean that it will in 5e. Though it can be helpful to look at old editions for inspiration, you should be careful of the rules environs of the edition you are trying to modify when making house rules.
No! The shield is not a weapon, this is designed to be a sword and board feature.
Shield is definitely not a weapon (no weapon stats), and there are currently no enchantments to make it a weapon. This is a feature designed for a brawler style (hand free for grappling) or a sword and board fighter.
Prone was an excellent choice
You said that the cleric wanted "to stop [the ghoul from] hitting people and make it easier to hit." The prone condition has the following two applicable features:
The creature has disadvantage on attack rolls.
An attack roll against the creature has advantage
if the attacker is within 5 feet of the creature. ...
While a shield can be indeed made of metal, you are wielding it, not wearing it.
In the equipment chapter (p. 144) the PHB refers to a wearer of armor, but says you wield a shield.
"Rules Answers: March 2016" from Wizards of the Coast points out that "various various game features distinguish between the armor you wear and a shield you wield." It provides ...
It's not that easy. Donning or doffing a shield costs an action: see the table on Player's Handbook page 146. Therefore most characters cannot don a shield after making an attack or doff the shield before the attack - they need a second action to do one of these, and two extra actions to do both.
The Animated Shield says that it
protects you as if you were wielding it.
This strongly implies that you aren't wielding it.
The Monk's Unarmored Defense feature works
while you are wearing no armor and not wielding a shield.
So the use of an Animated Shield shouldn't interfere.
As for proficiency, the only requirement for using an Animated Shield ...
A shield requires an arm, not just a hand, to wield:
Anyone can…strap a shield to an arm.
PH, Armor and Shields, p. 144
This jibes with real-world use of the large shields, where the arm provides necessary support to make the shield useful. (Bucklers, or hand shields, may be a different issue, but there are no bucklers in official D&D 5e rules.)
No (but your DM can allow it)
Player's Handbook, p.144:
You can benefit from only one shield at a time.
It doesn't say you only gain the AC bonus from one shield, it says you cannot benefit from more than one shield. If you attach a second shield, only one of your shields has effect (unless a more specific rule elsewhere overrides that).
Note that it ...
The relevant rule in the armor section states:
You can benefit from only one shield at a time.
This means that only one shield serves to give you any benefit (including the ones from the Shield Master feat).
This is further backed up from this Sage Advice page from the lead designer, Jeremy Crawford (thanks @V2Blast in the comments)
No matter ...
10% is the naive answer
The +2 bonus to AC is 10% of the d20 roll, but it is more complicated than that:
If the enemy can only hit you on a natural 20, adding a shield does not do anything
If without a shield the enemy hits you 50% of the time, it becomes 40% with one. The difference is about 20%1.
If the enemy only misses on 1, dropping the shield does ...
I think you're bypassing the "don" and "doff" rule for shields.
I agree that there's daylight to be seen between carrying and "wielding" a shield. But I'll pose you this question: if you can have a shield and switch states from "wearing" to "wielding" it freely, then what is donning or doffing?
As for the ...
Using armor or shields you are not proficient with has more drawbacks than only not being able to cast spells.
If you wear a shield without the proficiency you have...
[...] disadvantage on any ability check, saving
throw, or attack roll that involves Strength or Dexterity,
and you can't cast spells. (PHB p.144)
When you pick the War Caster feat you can...
A shield is an item. You can pick up an item without an action (Player's Handbook p.190).
Wielding a shield, not merely holding it, increases your Armor Class by 2 (Player's Handbook p.144). Donning a shield takes an action. (PHB p.146)
PHB p.146 says that armor must be donned to benefit from it.
If we wish to be exceptionally literal, p.144 says ...
A magic item meant to be worn must be donned in the intended fashion: boots go on the feet, gloves on the hands, hats and helmets on the head, and rings on the finger. Magic armor must be donned, a shield strapped to the arm, a cloak fastened about the shoulders. A weapon must be held in hand. (Pg. 140 DMG, emphasis mine.)
Also Rare Exceptions Exist. If the ...
Yes, you can cast spells with a Material (M) component without issue.
Your Holy Symbol takes care of it.
This is one of the more difficult things to look up in the current 5e rules. It starts on page 58, under Cleric spellcasting:
You can use a holy symbol (found in chapter 5) as a spellcasting focus for your cleric spells.
In the equipment section, page ...
The actual rules for this are oddly worded, but the notation is pretty simple.
In general, there are two formats for "things that increase your AC." The most common format is the one attached to armor:
[your AC equals] 11 + Dex modifier
or Draconic Resilience:
your AC equals 13 + your Dexterity modifier.
Note that there is no "plus&...
The rules for shields say:
Shields. A shield is made from wood or metal and is
carried in one hand. Wielding a shield increases your
Armor Class by 2. You can benefit from only one shield
at a time.
So as far as the rules are concerned, a shield only counts if you're carrying it in one hand. If it's on your back, it's effectively worthless.
No, wearing a shield does not count as Armor
Shields are in the Equipment section (Chapter 5) of the PHB, but are not classified under the Armor section when determining Light, Medium, or Heavy Armor.
The posted Sage Advice also contains some details differentiating Shields from Armor:
These methods—along with any others that give you a formula for ...
You can use a staff and shield, because you can use staves one-handed anyway
A quarterstaff is a simple melee weapon with the versatile property, meaning that you can wield it one-handed without penalty - it just does slightly more damage if you use it two-handed:
Versatile. This weapon can be used with one or two hands. A damage value in parentheses ...
You take the lowest of the two maximum dex bonuses.
As GMJoe noted, they aren't giving you a dex bonus, they're setting a maximum on your dexterity bonus to AC. As in, you cannot have a higher dexterity bonus to AC while using that item. If you have armor with a max dex of +1 and a shield with a max dex of +2, then the highest dexterity modifier your AC can ...
The only ways for a wizard to gain shield proficiency are either multiclassing, or the Moderately Armored feat. Since Moderately Armored requires you to be proficient in light armor, most wizards will need either a multiclass or another feat (Lightly Armored).
Note that you can use a shield without proficiency - you just won't be able to cast spells, and ...
Tower shields are not always bad. I've seen them used without being a hindrance (by someone who made a build around never having to make attack rolls). They're usually bad, though.
For one thing, it weighs 45 pounds. Not a problem for a 20 STR character, but it can by itself cause encumbrance for a weaker character.
It's armor check penalty is ...
In core folks do this with their tower shields
A creature fighting a dragon can take the ready action specifying the action as I gain total cover from my tower shield and the condition as When the dragon is about to use his breath weapon. When the condition's met and the dragon uses its breath weapon anyway, most dragons' breath weapons no longer have line ...
By strict RAW, no. Allowing it wouldn't harm your game meaningfully, however.
Shields are in the armour category, and are thus not weapons. You could easily argue that you could use it as an improvised weapon, but that does not make it an actual weapon. Using improvised weapons as justification for doing this does not hold up, as that would allow you to ...
It’s your shield, so it’s up to you.
There is no official “direct” answer to your question, except possibly what’s found in the introduction to the Player’s Handbook, in “How to Play the Game”:
The players describe what they want to do.
Sometimes one player speaks for the whole party, saying, “We’ll take the east door,” for example. Other times, different ...
Yes, because a shield is not armour
The shield in the equipment chapter is in a different category than light, medium, and heavy armour, and in the classes section shields are listed as a separate proficiency than each of the light, medium, and heavy armour.
Also PH page 14, Armor Class, mention wearing armour and carrying a shield several times. Note that ...
No, you can't load a hand crossbow while holding a shield. The errata added a new condition to the Ammunition property:
Loading a one handed
weapon requires a free hand.
A hand holding a shield is not a free hand, so you can't load your hand crossbow while using a shield. And since you can't load your hand crossbow, you can't make multiple attacks.
Too good when your offense doesn’t need hands
In theory, giving up a hand for more defense could be a fair trade—if you were losing offensive capability in the exchange. But as Dan B’s answer details, that just isn’t the case for far too many classes, particularly one your character would be likely to gravitate towards naturally, the cleric.
But maybe we ...