Sparked by a great idea in the comments of my previous post by dripton, I thought of some critical hit variations for GURPS. I haven't tested any of these.
On top of the "margin matters" idea from dripton, here are some options.
You Can Still Defend, But . . .
On a 3-4, you cannot defend against the critical hit unless that was exactly the roll needed to hit at all. Otherwise, you can defend at a -10.
Ex: You have Broadsword-15 and roll a 4 - the defender gets no defenses, roll on the critical hit table. You have an effective skill of Broadsword-4 and roll a 4. You critically hit, the opponent defends at -10.
Notes: Makes it possible to defend against non-3, below-hit-score-needed critical hits . . . but also means a lot more trips to the critical miss table for the defender! Dodging at -10 is a quick trip to the floor. You could say this doesn't affect critical failure, though.
You Can Still Defend
As above, but a non-3 or 4 critical hit is only a -5 to defend on top of any other penalties.
Notes: Basically you get a freebie Deceptive Attack if you critically hit. Much more likely to defend, but perfect shots are still perfect shots.
Roll, Roll Again
If you roll a 3 or 4 on a critical hit, your critical hit always has some special effect. Roll on the table, and re-roll any "No additional effect" results.
Notes: Criticals on a 5-6 are just no defenses, but on a 3-4 they're always special. Beware the "I need a 3 to hit" guys - either they really nail you or, nothing.
Basic Combat Plus
A 3 is maximum damage. A 4 is a table roll. 5-6 are just "no defenses."
Notes: This is a slightly souped up version of the basic combat system, which only rewards a 3 with any kind of bonus. Further note: I use the basic combat rules in my DF game to speed things up, because a table roll for a 1d+6 arrow or 3d+10 sword strike is just wasting time.
Just some ideas. No, I'm not trying any of these in my game any time soon. The last one is nice, but it's still an extra roll I'm not convinced we need.
Doug did something interesting with this idea, too.