Up to D&D
The terms “to hit” and “damage” in D&D are, I believe, misleading. If you change the “to hit” or damage rolls based on reasoning about the chance of landing an actual hit or doing actual damage, you are unlikely to get the results you wanted.
At first blush, it may seem like giving a character weilding two weapons two attacks is perfectly reasonable, but the results probably aren’t what you expect. Here’s a simple example to illustrate the point:
Let’s say Fafrd needs to roll a 7 or better to hit a bravo he’s battling. Let’s say that if he chooses to attack with two weapons, he gets two “to hit” rolls, but that he suffers a -4 penalty to both attacks. That means he needs to roll an 11 or better to hit, but he gets two attempts.
With one weapon, Fafrd has a 70% chance of hitting. With two weapons, Fafrd has a 50% chance of hitting, & two chances to do so.
|miss||2 misses||1 hit|
|hit||1 hit||2 hits|
That means he has a 75% chance of hitting at least once & a 25% chance of hitting twice! He’d be crazy not to always attack with two weapons.
The thing that’s really screwy, though, (& that’s hard to illustrate without lots of boring analysis) is that as you adjust Fafrd’s THAC0 or the bravo’s AC, the changes in the chances of hitting with two weapons v. one weapon do not change in an intuitive fashion. Playing with the penalty for two weapon fighting can “fix” a particular case, but you’ll just end up throwing off other cases. The problem isn’t the “offset”; the problem is “the shape of the curve”.
There are times when multiple attacks can make sense. In Wizards’s 3e D&D, at high level, a character’s chance to hit is so high, he almost always hits, which becomes boring. So, by giving the character an additional attack at a penalty, some of the fun of not knowing every attack is going to hit is restored.
This is dealing with an artifact of the game (that attack ability keeps going up while AC tends to level off) rather than trying to model something like two-weapon fighting.
(Depending on how you handle high-level play, classic D&D may or may not have the same problem. I haven’t gotten to high-level play yet...)
The criticism of iterative attacks is that, at very high level, there are too many of them.
What about monster attack routines? In many cases, the argument against multiple attacks for fighting with two weapons seems to apply here.
D&D has a lot of big, solitary monsters. (ahem...dragons) The D&D combat system (rightfully) tends to give the advantage to the side with more combatants, yet we want some monsters to provide more of a challenge even when they are alone. Giving these monsters multiple attacks per round is one way to address that.
For other monsters, though, I think their multiple attack routines aren’t really warranted. In some cases, I’ll just ignore the attack routines and only have the monster make one attack per round. For others, I only give them the next attack in their routine if the previous one hits. This tones it down without completely eliminating such an iconic part of the game.
There’s a long history of critical hits on a “natural 20” house rules. I’ve not be a fan of most of them.
One problem is the table of horrible things that happen on a critical hit. I took that part and used it for my injury table, which (normally) only gets used once hp are depleted.
The other problem is that they typically give a flat 5% chance of a critical. And if a monster’s chance of hitting is low enough, ever hit they make is a critical. 3e addressed this with a “confirmation roll”.
My solution is: A “natural 20” grants a “free attack”. Note that the extra damage from such free attacks does not necessarily mean that, in-game, more than one attacks were made.