One of the few things I miss from earlier editions of D&D is the excitement of rolling a crit as a GM. Don’t get me wrong, I love the fact that damage expressions in 4e are the result of a deliberate process, but there’s something quintessentially deflating about throwing a 20 and having nothing happen. Boredom and frustration kill GMs and this would seem to be an area where we can add some zest without upsetting balance.
You might ask “isn’t doing max damage enough?” I don’t think it is. Once your players reach a certain level of systems mastery seeing a crit is actually a good thing, as it establishes an upper bound to the amount of damage a monster can do. Encounters become much less exciting once the players know exactly how bad things can become.
There’s also the disparity between character and monster criticals. It’s perfectly viable for a character to build for more crits and/or bigger crits. Barbarians are famous for this (yes, I’m ignoring the degenerate Avenger “crit fisher” build). Further, every character gradually improves their critical by gathering more powerful weapons and implements. Monster crits remain largely static.
The simplest mechanic is to give every monster has high crit (the attack does the maximum damage it would normally do plus the result of the dice that would have been rolled. 3d10+6 becomes 36+3d10, for example). While this has the virtue of being simple, I don’t think it achieves the goal. Neither does a system of multiplying damage, which would just be added complexity.
I came up with the following chart as a simple start:
|16-17||Push equal to STR mod and knock prone|
|18-19||Dazed (save ends)|
|20||Stunned (save ends)|
I think it does a pretty good job of “shaking things up” without making combat too swingy.