Today I have to say that I think dexterity, weapon speed, and casting time are the most useful tools to get a more realistic response time.
Back in 1st edition I never ever messed with such however, typically just a 10 sided die was rolled by each character and people went in the order of the flat number. By the time I started playing 2nd edition, I started using dex bonus to adjust but most of the time nothing else.
The reason I think so many people skipped weapon speed factors in 1E and 2E AD&D is that the rule is implemented in a quirky manner.
IIRC (those playing 1E AD&D correct me), the character with the longer weapon always attacked first on the first round of combat, and then on the character's initiative for his side. Weapon Speed Factors were used to break ties, and when they were used, a smaller weapon could gain extra attacks if its SF was sufficiently smaller than the SF of his opponent.
With 2E AD&D, the Speed Factor rule was simplified a bit in that it was used as a straight modifier to initiative.
In 3E, Speed Factors were thrown out of the game.
The idea behind the rule is to account for size, weight, encumbrance, and the speed at which a character can wield a weapon. To some extent, it's also about weapon length. For example, a character wielding a dagger might be able to zip in and jab his pointed edge at the enemy several times as his opponent makes one swing of a big two-handed weapon. But, the dagger user has got to get in close enough to use his weapon--and that's not easy when your enemy is swinging a long two handed hammer at you.
The CONAN RPG approaches this idea from a different angle. The rule is simple and easily portable to D&D games. There's no fuss as there was with the Speed Factors of old.
The rule is this:
For every size category a weapon is larger than his opponent's weapon, the attacker gets a +1 Parry Defense modifier and the defender must use a -1 Parry Defense modifier.
For example, let's say one character uses a dagger and his foe uses a two-handed sword. The two handed sword is two steps bigger per size category than the dagger (The dagger is a Lite weapon. Then, there's One-Handed weapons, and the next category is Two-Handed weapons).
This means that, when the dagger character defends, he is -2 to his Parry AC. And, when the two-handed charactger defends, he is +2 to his Parry AC.
Note that in Conan, a character also has the option to Dodge, which is a separate defence AC. Thus, in the situation just described: Let's say the dagger user has a Parry AC 14 and Dodge AC 13. This modifier reduces the Parry AC but not the Dodge AC, thus the character is AC 12 when parrying the two-handed sword and AC 13 when Dodging the same attacke--therefore the dagger user will Dodge the two-handed sword user and not try to parry his big weapon.