I was thinking about this today, and I'm hoping this may work. A few players I have talked to, remember the days when a natural 20 meant automatic double-damage. Ahhhhh .... such easier days. As we all know, when you score a critical, you have to roll a 2nd time to see if you get the x2, x3, etc., modifer from your weapon of choice. Well as I was saying, some of the players I know, have gotten very frustrated with that rule from the D20 system. So I'm thinking this:
For this example, I'll use your standard Fighter, equipped with your standard, (non-magical,) long sword. (Think level 2, character is just starting out.)
A roll of a natural 19-20, is a critical chance with a long sword. My house rule is, on a natural 19, the player rolls the 2nd roll to see if it's a critical hit. On a natural 20, it's an automatic x2 to damage. No second roll needed. Now the kicker is, let's say the Fighter is using a Great Axe. With the great axe, it's critical is x3, and a natural 20 is required for the critical chance. In this case, on a natural 20, the player gets a choice. Don't roll the second time, and you get an automatic x2 to damage, OR, roll the second roll, hit for x3 or risk missing and having it just be normal damage.
The other part of the rule is, let's say your a wizard with a quarterstaff, your fighter is laying on the floor bleeding out. The monster comes at you and hits your weapon and breaks it in half, (for simplicity's sake, it's non-magical quarterstaff ..... level 2 wizard.) With his dying breath, the fighter kicks his long sword to the wizard because the wizzy can't escape right away. The above rule will not apply to the wiz, because he is not proficient with the sword. If he does roll a natural 19 or 20, the normal crit rule applies.
That's what I got so far. I haven't figured out how I can apply this to the druid's natural attacks, or the monk's unarmed strikes. Other than maybe unbalancing things, I also do not want to take away from the Weapon Mastery ability that Fighters get at level 20 per the PF Core Rulebook. Give me your thoughts, ideas, and feedback.