My version of criticals are very homebrew. They combine some elements from 4e (such as maximum damage, instead of double/triple damage), along with some new stuff (such as criticals for spells), and of course, critical failures.
Combat. A Natural 20 in combat means an automatic hit, regardless of armor class. If the 20 + the Attack Bonus also hits the AC, then it is a Critical Hit, and maximum damage is inflicted.
Spells. When casting a spell which has a touch or ranged attack, spells are able to critical, by having the magical energies channel into a vulnerable area of the target, just like a weapon-based critical hit. Maximum damage is inflicted.
A Natural 1 is an automatic failure, regardless of your modifier. The result is completely random, at DM discretion and will generally be based on the current party activities.