Thought: In a game where a d20 is rolled for many different things besides to-hit, such as skill checks, initiative, and many other reasons (including random boredness) and crits ONLY affect to-hit rolls, then the percentage of crits is actually less than 5%.
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Also, and in contradiction to the above, I think EVERY d20 die roll, even initiative and skill checks, should have the option to "go crit" on a 20.
Depending on what you mean by "go crit", that was true for some other rolls in 3e. Almost every check other than a skill roll was automatically successful on a 20. Spells critted when the enemy rolled 1 on a save.
In addition to what has been done before, I would consider making saves crit so that save-halves effects allow a second save on a natural 20 to avoid even the partial effect. Skills shouldn't crit by default, since the effect can be anything and the check shouldn't always succeed even on a natural 20.