It may seem to go against the other things I've posted in this series, but I like critical hits. Even though they may slow a game down, they inject an extra degree of flavour into the narrative.
Critical hits don't happen all the time, and that's one of the things that makes them interesting. Worked carefully into a system the open up a range of possibilities, but when ad-hoc shoved into an otherwise elegant system they can simply seem like an annoying diversion from the real action.
Different games handle the mechanism of the "Critical Hit" in different ways. The classic old-school version said that if you roll a natural 20 (that's a roll of 20 on the 20-sided die, before any modifiers are applied) then a critical hit occurred. A critical hit might do maximum damage, it might do double the damage rolled, or it might have an effect based off a table.
3rd Edition D&D started messing around with this formula by creating threat ranges; where if you roll naturally within a certain range of high figures, you've got a chance of getting a critical, then you have to actually roll again to achieve the elusive critical. Again, depending on the player options used, the actual critical might do double (or triple damage), or might have an effect from a table.
The Fantasy Forge Star Wars Beta, has just updated it's critical hit rules, before they applied some fairly vanilla modifiers to a victim, but now they run of a varied table of mysterious injuries that could be applied. In this system you can gain successes or advantages during your die rolls; a success applies damage, reaching a certain threshold of advantages allows a player the chance to roll on the critical hit table (different weapons have different advantage thresholds).
As examples from the worlds of miniatures, Mordheim uses d6's and allows a critical hit if you roll a natural 6 on your injury roll. Critical hits have a secondary table with three results, each of which deal extra damage, and some of which make it impossible for the victim to absorb that damage regardless of their armour. Rackham's Confrontation used a d6 system also, but you rolled 2d6 (the high result determining damage, the low result determining where you hit) doubles on damage rolls applied critical hits.
Some games even include "critical hits" for non-combat situations. Perhaps enough of a success in the social arena might shut someone down for the rest of the scene while they deal with their wounded pride. Perhaps a crafting action might produce an unexpected masterpiece.
I guess that critical hits were one of the first ways to create a system with multiple degrees of success. Either you missed, you hit through skill or you got incredibly lucky and did something massive.
Critical hits provide an occasional bonus, they aren't something you can evenly rely on, but when they do manifest in play, they provide that kind of storytelling moment that great gaming anecdotes are made of. Critical hits can also provide depth and colour to the attacks made, a few game systems use a variety of critical hit tables to reflect the capacities of different weapon types and combat styles (one for slashing effects, one for piercing effects, one for bludgeoning...etc.).
As long as critical hits are used sparingly and the effects produced are complimentary to the story, they can be great.
One of the issues with critical hits is that they often tend to be a subsystem only tenuously linked to other parts of the game; this forms a bit of a disconnect when the critical rules are activated during play. Any break in procedure can cause a moment when the game and the story lose momentum...like those times when a rule book is pulled out for a clarification. If that pause makes for an appropriate moment of tension building, then it might not be so bad; but if the pause is simply because of confusion, or over-complication, then it becomes hard to regain the pace of the session.
Further cons come when the critical system doesn't feed back into the other mechanisms or narrative effectively, or when the benefits of the critical hit aren't that special. The first incarnation of critical hits in the new Fantasy Forge Star Wars game were an example of this; they may have been simple andtied into the other mechanisms, but they didn't really do much. The thrill of the awesome roll was negated by the fact that the roll didn't have an awesome effect.
I like the idea behind critical hits, but they need to be implemented correctly.
A bit like some good spices in a meal...too many and they lose their impact, not enough and you don't even realise they're there, the wrong type and the meal just leaves a bad taste in your mouth\.