skotothalamos
formerly roadtoad
The confirmation roll on critical hits. I absolutely hate when I can only be hit on a natural 20, so every single attack that hits me deals double damage.
You could house-rule in the 4e version of the crit rule, where a natural 20 is only a crit if the total roll would be a hit; if the bonus+20 is still shy of the AC, all the 20 does is make it into a normal hit.