In 3E D&D, a keen
weapon's increased threat range stacks with the increased threat range
from the Improved Critical feat. In 3.5, this is no longer the case.
Some think that this rule was made because the combo is too good, especially with creatures with a lot of bonus damage (which gets multiplied along with the crit). According to some, the change was made because having a crit range that's too big means crits happen to often, and are therefore no longer "special."
I respectfully disagree on both points.
Update 2/18/04: I've decided to redo this rant, as it's sorta scattered and makes some assumptions that are correct but some people aren't comfortable with. The first step is to do a comparison of longsword and rapier and how keen and Improved Critical affect their damage, and compare that to the effects of a simple energy property (such as flaming) or the Weapon Specialization feat. Eventually I'll do other comparisons and fold them back into this main document. In the meantime, please accept my apologies for directing you through multiple documents for this rant.
Is the Combo Too Good?
For this part, I'll prove
numerically that allowing these crits to stack is necessary for the
high-threat-range weapons (such as rapier and falchion) to keep up with
normal-threat-range weapons, and if it's not egregious in the case of
frequent crits, it isn't a problem with infrequent crits.
As a player of a character with this combo in Monte's game, I played alongside Erik Mona's paladin Zophas, who was lower-level than my character Shurrin (but our net attack bonuses were almost the same due to Weapon Focus and some levels in cleric and rogue that I had). Shurrin used a +2 keen rapier. Zophas used a +1 holy greatsword. Not counting Strength bonuses (and his was higher than mine, so it only would skew the example in his favor more, and he'd get x1.5 for using a two-handed weapon, which would make it even better for him), here's how it went:
Against nonevil opponents, Zophas was doing 2d6+1 damage (8 points on average) with a 10% chance of a crit (so +10% x 8 points, or +.8 points), for a total of 8.8 points per attack. Shurrin was doing 1d6+2 points of damage (5.5) with a 45% chance of a crit (so +45% x 5.5 points, or +2.475 points), for a total of 7.975 points per attack.
So right from the get-go, his character (with essentially the same attack bonus) is doing more damage per round than my character. And that's on the primary attack ... on iterative attacks we're both less likely to confirm criticals; since his base damage (not including crits) is higher than mine, it means his average damage is higher for these attacks (8 points vs. 5.5 points).
It gets really obscene when we're fighting evil opponents. His weapon is dealing at extra +7 damage per attack (15.8 total), while mine deals no extra damage. I'll remind you here that our weapons have identical plus-equivalent values (+1 enhancement +2 holy = +3; +2 enhancement +1 keen = +3).
And my weapon doesn't get its "bonus" (crit) damage against undead, so when fighting undead we're back to comparing the 8 points vs. 5.5 points. Note that most undead are evil, so he's probably also getting his holy bonus on top of that.
And then you have to consider that I paid a feat for Improved Crit just to keep up with his weapon damage, and I'm still behind in the average and the optimal situation.
For those who point out that the falchion is a better version of the rapier, let's run the numbers for the falchion.
If Shurrin had a +2 keen falchion instead of a +2 keen rapier (and Improved Crit with falchion instead of rapier, of course), his average base damage would be 7 instead of 5.5. With crits included his average damage would be (45% x 7 = +3.15) 10.15 (compared to Zophas' 8.8). Iterative attacks are still less likely to crit, same situation as the rapier, so this advantage essentially disappears after the primary attack (Shurrin's 7 vs. Zophas' 8). So against nonevil crittable opponents on the first attack, Shurrin would average +1.35 points ahead of Zophas. Against evil crittable opponents Zophas again jumps to 15.8 and Shurrin is still at 10.15, and Zophas still has the advantage against uncrittable things.
So with weapons of equal value, Shurrin is a little ahead against nonevil creatures, behind on uncrittable nonevil creatures (just like with the rapier), and far behind against evil creatures (just like the rapier). This despite the fact that Shurrin had to spend a feat in order to get his advantageous crit threat range.
[Aside: Let's compare the absolute base values of a nonmagical greatsword and falchion. Greatsword = 3.5+3.5 = 7, with crits = 7.7. Falchion = 2.5+2.5 = 5, with crits = 5.75. So falchion is worse. Include keen or Improved Critical on the falchion and you add another threat increment of .75 damage, for a total of 6.5, which is still worse than the greatsword. Add another threat increment (if you have keen _and_ Improved Crit _and_ they stack) and you're at 7.25 damage ... still not as good as the greatsword! The only way Shurrin's hypothetical falchion is keeping up in base damage is that his falchion is +2 while Zophas' greatsword is only +1. The falchion is really a worse weapon than the greatsword. Likewise, the rapier is worse than the longsword.]
[Another aside: Let's be totally crazy and assume Shurrin and Zophas
both have 20 Str (and we'll take into account the x1.5 Str bonus -- 7.5 points -- for
the weapons being two-handed in this example, since that might be
enough to help Shurrin come out on top). Now Zophas' +1 holy greatsword
averages 7base+1enhancement+7.5str = 15.5 base damage, 17.05 including crits. Shurrin's
+2 keen falchion averages
5base+2enhancement+7.5str = 14.5 base damage, 21.025 including crits.
Shurrin's ahead by almost 4 points -- woohoo!. Against evil creatures,
Zophas' damage still jumps by 7 to 24.05, and against uncrittable
things Shurrin's damage drops to the base value of 14.5. So with a
hyped-up Strength, which is where the argument for this rule revision
comes from, the falchion-wielder is just barely ahead of the
greatsword-wielder in ideal circumstances (and note he had to spend one
more feat than the greatsword-wielder to get there), and in non-ideal
situations (such as when he can't crit, or when fighting evil
creatures, or both) he still loses compared to
the greatsword-wielder, even though he spent more money (25gp) on his weapon
and spent one more feat (Improved Crit) than the greatsword-wielder.]
Does It Make Crits Un-Special?
Imagine an average 10th-level PC fighter. He has a +10 BAB, and say
a 20 Strength (base + magic). He's spent almost all of his 49,000 gp on his
weapon, a +3 keen rapier
(32,000 gp ... and I don't have data on how much a typical fighter
spends on his weapon, but I'm guessing it's not "almost all"). Assuming
he's not using Weapon Finesse, his total primary attack bonus is
+10+5+3 = +18. With Improved Critical, he threatens a crit on a 12+.
If you look at a range of CR10 and CR11 creatures (and only the ones
that can be critted), their ACs are in the 20-30 range. That means that
at worst (AC 30) the fighter has a 50% chance to hit, and about a 40%
chance to threaten; if he hits, he has to confirm (about a 50% chance),
so that means his odds of hitting and confirming are (50% x 40%) = 20%. So on average he's just about guaranteed a crit if he fights a
creature for five rounds. And if he crits, he does an extra d6. Woohoo.
So maybe letting keen and Improved Critical means crits come up a lot more often. Maybe that makes crits a little
less special. But we're talking about crits that are an extra weapon die
of damage ... we're not talking about Arduin-style crits that do 1d100
damage, or sever limbs, or do instant-kills ... it's just another d6 or
d8. They're not that special in the first place. And as established
above, the weapons that crit more often need those crits to keep up with standard-crit-range weapons, otherwise there's no real point in choosing those high-threat weapons.
And there are a lot of things in the game that are "special."
9th-level spells are special. Paladins are special. Dragons are
special. But we allow 9th-level spells on scrolls. And we let paladins
as PCs as commonly as fighters (there are no mechanical restrictions on
how often you can have paladin PCs). And we now have little wyrmling
CR2 dragons in addition to the big hulking party-killer monsters. Don't
those things make 9th-level spells, paladins, and dragons less special? Don't the epic-level rules make everything under 20th level less special?
"It's not special" isn't really a good reason to cut a mechanically
sound mechanic from the game. It's a reason to consider
cutting it, but the benefit of leaving it in (as demonstrated above) is
necessary for that character build and choice of weapon, and isn't that damaging to the "specialness" of crits.
For the lower-damage but higher-threat-range weapons to be viable in combat
(as they were designed to be by the original core designers, who crunched
all the numbers), keen has to stack with Improved Critical.
Otherwise there is no game-mechanical reason to spend the extra 25 gp
for the 2d4 falchion instead of the 2d6 greatsword, or the extra 5 gp
for the 1d6 rapier instead of the 1d8 longsword, or the extra feat to
increase your threat range ... choosing those high-threat weapons is
just a bad choice. And if letting these mechanics stack makes crits a
little less special, that's only a small mark against letting them
So let them stack, dagnabbit.