When the game was first announced in 2008, it was revealed that crits would have dealt very different effects depending on their elemental damage type. This feature was removed during development, but the original plan went as follows:
Physical Damage: Deals double damage and possibly knockback.
Arcane Damage: Critical hits from arcane damage silence targets for 4 secs. (Silenced targets can not cast some spells, such as a Skeletal Summoner or Goatman Shaman resurrecting fallen minions.)
Cold Damage: Critical hits from cold damage freeze targets for 2 seconds.
Fire Damage: Critical hits from fire damage set targets ablaze, dealing additional damage per second.
Lightning Damage: Critical hits from lightning damage stun targets for 2 seconds.
Toxic Damage: Critical hits from toxic (poison) damage had an unknown bonus. Extra damage over time?
These functions were removed along with the entire system of elemental damage from characters, and all critical hits became hits that added (at base) 50% damage. There are no special secondary effects (other than visual) to critical hits in Diablo III, though many skills trigger procs when crits are scored.
- See more at: http://www.diablowiki.net/Critical_hit#sthash.Hf1pD24p.dpuf