It concerns me that there's next to nothing that a knife-wielder or an archer can do to really deal damage with a single attack. The case in point above was a Rogue, who has the SA option, but a knife Fighter can do no more than 1d4+str with a dagger (you can't even use PA with light weapons). This is not only unrealistic, it does a poor job at modeling cinematic combat. Think about Benicio del Toro's character in The Hunted. A skilled fighter can kill someone with a knife, with one or two precision strikes.
I've been toying with a variant WP/VP system. I know everyone seems to have a big problem with WP/VP, but since I haven't played SWd20, I guess they're not ruined for me. The system would work this way: if the attack roll beats a targets AC (Defense Class, in my system) by a margin of 20, the attack is a critical hit. Weapons with a higher crit range provide an attack bonus. For instance, daggers have a +1 to hit. Critical hits would do CON damage. Weapons with a x2 multiplier would deal normal damage (i.e. a longsword still deals 1d8, but it's reduced from CON). Weapons with x3 critical multipliers would deal x2 damage instead, and weapons with x4 multipliers would deal x3.
Keep in mind that this system would work in conjunction with other variant systems. Armor would give a DR value equal to its armor bonus, and would have no effect on AC (crits would bypass DR). PCs would be granted a Class Defense Bonus (based on REF saves, not BAB). The system is still very experimental, but it's aimed at solving these problems: excessively bold players who fight before they think, a single blow not being able to take down a character, and an inability to model realistic or cinematic combat. Please, give me constructive criticism.