+1 to increased crit range for Adamantine Weapons. Expanded crit ranges are a cool perk that aren't seen that often in 5e. Also I'd be in favor of different adamantine weapons having different effects. Weapon differences are also sorely lacking in 5e.
-1 that Adamantine Weapons auto crit. I agree that that'd be too powerful, especially when Paladins are released.
+1 that normal Helmets don't negate crits. That's very powerful for a commonly found item. Personally I'd want them to have tradeoffs: e.g., reduce crit damage by 25% AND impose a -2 penalty to Perception.
+1 that Adamantine Armor has some type of crit negation and/or AC boost and/or Shove resistance.
