Lets face it, they add excitement for the players, but they are also a hidden albatross, wearing down the players' hit points more over the long term than the cannon fodder they are dispatching.
Additionally, they tend to be a fixed number. Natural 20 is a common on (you crit 5% of the time, whether your chance to hit is 50% or 5%), or its a range of numbers, usually depending on weapon (thinking 3x here), something like 18-20.
What is the usual critical effect? Maximum damage or double damage. Why, because it's simple to figure out.
Here's my thoughts (not that I'm finished thinking) on the matter.
Why don't we make it range dependent upon the number you need to hit. Something like "all rolls 5 or better than the score you need to hit may be a critical. Please refer to the critical table". This means the better your chance to hit your opponent, the better your chance to possibly critically hit him. Additionally, if you barely have a chance to hit him, you have no chance to score a critical.
Critical Table (Roll 1D10)
1-3 Normal Damage only4-5 +2 To Hit on your next attack against your opponent (You are pressing your opponent)
6-7 +2 To You AC until the end of your opponent's next round (You have your opponent off balance)
8-9 Get the benefits of the above two rolls (+2 Hit and +2 AC against your opponent)
10 You have an opening! Get a free attack immediately against your opponent! Your free attack may also be a possible critical if you roll high enough.
This does away with automatically adding damage to the attack, while still offering bonuses for the player (or the monster).