Critical Hits:
Max Damage that attack would do, then roll damage die again.

Critical Misses:
Situational. If attacks and friendlies near by, possible hit on them, during skills (even though you can't critical fail a skill) possible recourse. If no friendlies around, depending on how you planned attacking, you may loose your grip on the weapon or fall down and such.

Additionally.
Critical hits do not need to crit confirm. A natural 20 or whatever results as a crit for your character automatically confirms.