thecasualoblivion
First Post
It always has worked out that the higher your AC was, the more impact an extra +1 AC would have. If at your AC (whatever it is) vs a particular enemy you have 10% chance of being hit, adding 1 AC lowers it to 5% - meaning you effectively cut incoming damage by half! However, if at your AC you have 75% chance of being hit... having 80% or 70% is not really significant, the impact on your survivability.
Because of how the numbers in D&D work out, it's not vs "a particular enemy" anymore but rather "enemies in general", since as you point out the average monster has a pretty narrow range of +to hit numbers. You are very correct that it's now possible to make judgement calls about AC values in general.
I'm not sure however if I agree that there is a deadzone. Rather as above, it's "the lower the more worthless each AC point is". Maybe what you are alluding at is a "zone of effort"? It's usually somewhat trivial to increase your AC from 10 to 14-16 with a bit of dex, a bit of armor/shield, a defensive spell etc. So it's worth it to increase your AC for such little effort.
So if McAverage McMonsterface has +5 to hit, going from AC 11 to AC 16 lowers your chance of being hit from 75% to 50% - that 1/3 less damage coming in, that's definitely worth it. But is there a "deadzone"? It might be more difficult to go from 16 to 18 than from 14 to 16. But doing so will decrease your chances of being hit from 50% to 40% - a 20% reduction in effective damage taken. I think that's still worth it.
I'm still learning the rules but the loss of tank "stickiness" is a problem IMO.
I played a few games last year at 5th level, with a plate wearing Cleric with 20 AC who cast Shield of Faith on himself to buff that to 22. Surrounded by three enemies, he didn't last long. With the exception of a raging Barbarian with high HP taking half damage from weapons(or everything), it's almost impossible to survive focus fire in this game from what I've seen. It doesn't matter if you're sticky if you can't survive focus fire.
There are a few ways to get stickiness. One is the sentinel feat, fighters have a fighting style and Battlemaster maneuvers, Paladin has something as well I forget how it works, and ironically the most powerful one belongs to the Swashbuckler Rogue. None of them really stack up well compared to what a level 1 defender can do in 4E, and unlike 4E you have to trade feats, damage dealing ability, and specific class levels for the privilege.