Critical Strike is executed at +5 attack. If the strike connects, it is an automatic critical hit with a 20% chance of killing non-boss opponent outright if its health is equal to or below 20%. Source: talent_constants_h.nss.
Conjuration time: 1s.
80 additional threat is supposed to be drawn from the target. As of patch 1.02, only damage-based threat is drawn.
Activation base is in stamina and values vary with fatigue%.
Mighty Blow is a tier 1 talent with a 20s cooldown, that hits at +10, causes an auto-crit, and staggers enemies, interrupting their melee auto-attacks (it won't interrupt any talents or spells). Critical Strike is a tier 4 talent with a 60s cooldown, that hits at +5, causes an auto-crit, and has a 20% chance to finish off normal or lieutenant rank enemies below 20% hp. This comparison speaks for itself. A lieutenant rank creature under 20% hp that would warrant a Critical Strike attempt (instead of using Sunder Arms, for example) is yet to be discovered. The talent is useful as the second auto-crit when Mighty Blow fails to stun or you just need to deal a lot of damage as fast as possible, though.