Last Guy in the Airlock
To rephrase what I was saying, people that support the "AC = 16 minimun including all modifiers" position take the position that the "AC" in the barkskin spell descripition, means "AC including all modifiers". What I'm saying is that:
1) in the armor chart, there is a numerica value for AC than will later be modified by DEX modifiers, shields and other modifiers such as cover.
2) the text of the barkskin spell specifically mentions that the mimimum AC value will be 16 regardless of what type of armor the target is wearing. The text makes no reference to "regardless of shields, DEX mods, cover or other modifiers".
To me, the specific including of armor only in (2) and the exclusion of all other bonuses, then points towards interpretating the AC in the barkskin spell description in the way I mention in (1).
This is a good statement of your position (and Defcon's, as I understand it). It does make better sense of the spell, but it does not match the definition of armour class in the PHB -- p. 14 -- where the armour class is your base (10 or what armour gives you) + your shield + your dex. (and we know that, with specific trumping general, it will include con/wis bonuses from barb/monk, draconic sorcerer protection, etc.)
It is the sum of these things that are your armour class, because (p. 7) the armour class is defined as the target number.
Two things follow from this:
1. The rule makes good mechanical sense, but cannot be easily explained by in-world physics especially if you literalize "barkskin" and your skin gets a bit thicker when you drop your shield). We can find similar holes throughout the game (hit points and healing, anyone?), and as a result I do not see this as fatal.
2. While my reading includes all abilities, magic rings, and armour including shields, I do think a reasonable case can be made for "DM's call" in two cases:
a. parrying (and similar maneuvers, that expend a resource), I'd probably allow to add to the AC at 16;
b. cover too. Cover is external to the character and is something that certain abilities negate (e.g. sharpshooter feat).
Both of these are situational bonuses to your AC, that opponents have an opportunity to bypass.
But in the end it remains unclear, and we each feel our own reading is the most natural one.