What If: crit when beat AC by 5 or more

[Goobahfish]

One issue I can see for a simple physical DR swap is that it badly disadvantages heavy armour users against spells. 5e doesn't distinguish between "touch" and armoured AC the way 3.5 did: the AC that the game uses to protect you from a swing of a sword is the same AC it uses to protect you from a Fire Bolt. Save-based spells are obviously unaffected by this, but any spell attacks that don't do b/p/s (which, realistically, is most of them) have now gotten a big accuracy boost without any drawbacks.

This all depends on how spells are handled. If attack spells go back to being saves (i.e. a Dex save) this resolves itself pretty well. The other question is whether armour protects against fire/acid/lightning damage etc. There is a pretty good argument on all accounts here (save perhaps lightning - though if you are wearing gambeson...).