The current mechanism is simple, but somewhat unsatisfying: a natural 3 or 4 on an attack roll is a critical hit, and the opponent gets no defense roll. A natural 17 or 18 on an attack roll is a critical miss, and something bad happens to the attacker. An attacker with an effective skill of 15 or better also gets a critical hit on a natural 5. An attacker with an effective skill of 16 also gets a critical hit on a 6 and doesn't critically miss on a 17, though she still misses on a 17.
There's about a 5% of rolling a 5 or less on 3 dice, and about a 10% chance of rolling a 6 or less. So going from effective skill 15 to 16 only slightly increases the chance of hitting, but it doubles the number of critical hits. Against an opponent with significant defenses, that can increase the chance of getting through the defenses by 5%-10% on its own. And attacking at skill 16 also reduces the chance of critical failing from 2% to 0.5%, which is either not much of a difference or a fairly huge difference. A rare event goes from happening one time in fifty to one time in two hundred.
The reason that all this is unsatisfactory is that it makes optimizing fairly simple: attack with skill 16. Going below 16 can slightly increase the chance of getting an attack through defenses, at the cost of giving up potential critical hits (and their beneficial effects such as extra damage or bonus armor penetration) and risking more critical misses. There are circumstances where going to skill 14 increases the chance of getting a hit through defenses slightly (going from 37% to 42% or so in the most favorable case) but most people feel the extra risk of critical misses isn't worth the slight gain in some circumstances.
I wish the optimization wasn't so simple, and there were more situations where having an effective skill ranging between 14 and 18 were valid but different. Different players could make different choices, depending on their preferences and the tactical situation, without feeling that they were making an inferior choice.
Proposed House RuleMy rule is a simple modification of the existing critical hit rules. An attacker with an effective skill of 15 or better also gets a critical hit on a natural 5. An attacker with an effective skill of 18 or better would also get a critical hit on a natural 6. An attacker with an effective skill of 14 or better wouldn't critically miss on a 17.
It's just a simple tweak to the current rules, but I think it opens up some space in play. A moderately skilled attacker can drop her effective skill to 14 to rack up penalties on the defense without increasing her risk of critical misses. Or if she wants to maximize her chances of critical hits, she can raise her effective skill to 18 and go crit fishing. Depending on the situation, she may even want to hold at 16, maximizing her chance of a hit without getting critical: useful when she wants to draw out a defense to set up another character or has an attack that penalizes the defender for successfully defending (such as a lightsaber or some other annihilating weapon).