Rules - in role-playing games at least - are open to discussion, and here is a fascinating offering concerning the whole area of critical hits. The rules as they stand 'merely' allow for increased damage when you land a critical hit. Now for those to whom combat is a means to an end, that may be sufficient; but a lot of gamers relish combat as an important element of their game and they at least ought to check this out. For that matter, there's plenty for those who prefer in-depth role-play over 'roll-play'...
The methodology for achieving a critical hit is left unchanged. Roll that '20' and then confirm the critical with a second roll using the same modifiers, which must at least hit the target. Then it gets interesting, with a range of tables to consult based on the sort of damage that is being inflicted. Users of magical weapons that deal bonus energy-based damage may choose a table relating to the energy in question instead. Furthermore, to allow greater customisation of what happens, there are suggestions regarding damage modifiers, severity (a weapon with a higher modifier causes a more serious wound) and the type of weapon being used (for example, a light one does less harm than a heavy one...).
As a bonus, there are also some feats and spells specifically related to the topic of critical hits that may be incorporated into your character (or NPC/monster) build. These cover both defence against criticals as well as enhancements to your chances of inflicting them. And there are a few magic items abiities to apply to your cherished weapons too.
And on to the tables. These hark back to the days of the wonderful damage tables in Iron Crown Enterprise's "Arms Law" rules, plundered for house-ruling into many other systems in my groups and I'm sure many others as well. So a piercing critical may result in an eye poke or just a pin-prick (but one that won't stop bleeding!) whereas your bludgeoning attack, if it lands just right, might break a foe's back!
Bring it on!