Critical hits should certainly exist. They're just fun. My preferences:
- A natural 20 (only) is an automatic hit, and a critical threat (as in 3e). You then proceed with the confirmation roll.
- If the confirmation roll fails, you do normal damage. If the confirmation roll succeeds, you do critical damage.
- Critical damage should generally be approx double damage. Whether this is literally a case of rolling the damage twice (3e), or whether it's max damage (4e) doesn't matter, but it should be about that. The 3e x3/x4 critical multipliers should be eliminated (as indeed any threat ranges - crit on nat-20 only!), as should those mundane weapons and feats that do extra damage on a crit.
- However, some powers and/or magic items and/or combat maneuvers should allow the possibility of the character forgoing the extra damage from a crit, and instead triggering some other effect instead. Indeed, for such powers and effects, there's not necessarily a need for the confirmation roll to be the same as the original attack roll.
Some examples:
The Fighter attacks with his rapier. He scores a natural 20, and decides to go for a disarm. Therefore, his confirmation roll is an attack vs Reflex, and if he succeeds he does normal (not critical) damage, and his target is disarmed.
The Rogue makes a sneak attack. He scores a natural 20, but decides to just go for maximum damage, rather than trigger his hamstring option. Thus, he makes a confirmation roll, and if successful does double damage (including all those lovely d6s from his sneak!).
The Wizard casts disintegrate. The initial attack is against Reflex, and does 8d6 damage. He scores a natural 20. Now, he can decide to confirm for a further 8d6 damage if he wishes, but the disintegrate spell allows him to instead confirm for a further 4d6 damage and turns it into a Save-or-Die. The wizard gleefully goes for the second option, rolling his confirmation against Reflex again...
I believe having confirmation rolls of this sort opens up a bigger design space (since the confirm doesn't need to be the same as the basic attack), it is conceptually a bit cleaner (since you're less likely to crit a guy in plate than one in leather), and it allows for crits to be that bit more devastating when they do happen.
Plus, allowing a confirmed crit to become a Save-or-Die means that a SoD spell has about a 1% chance of killing outright (5% of nat-20, then approx 70% of confirm, then 45% on the save) which feels about right, and avoids them being "all or nothing" effects (since you always get the normal damage anyway).