Speed Factors and Weapons vs. Armor
In 1st edition AD&D I recall there were two rules we pretty much totally ignored (well there were probably more than two...) These were rules for weapon speed factor and for weapons getting different bonuses and penalties vs. different types of armor.
These were decent ideas in theory but in practice we really could not makes heads or tails of the rules. That's not entirely true - we couldn't make heads or tails of the speed factor rules.
In theory this was a good idea. In practice it was a problematic rule. First of all the table was indexed by Armor Class. This assumed the AC was unmodified by magic, dexterity, etc., something that was rarely true. Secondly it required referring to the table every time a weapon was used, slowing down game-play. Thirdly it broke down when using monsters. Is a mace or sword more effective vs. a dragon's scales? And do natural attacks get modifiers? Shouldn't they?
This is something that I liked the idea of but I don't think AD&D got right (as I recall this is something that was also in the original edition rules, starting with Greyhawk I believe.) AD&D 2nd edition handled this better in my opinion. As I recall, every type of armor received bonuses and penalties vs. three types of attacks - blunt, piercing, and slashing. Each weapon listed what types of attacks it could make. Not quite as realistic but in my experience this was far more playable.
The best handling of this, in my opinion, is the way the Rolemaster RPG handles this. A large portion of the game is dedicated to tables of weapons vs. armor but cleverly it is the only table you need to refer to. You roll percentile dice, add your offensive bonus, subtract your foe's defensive bonus (which does not include armor), and refer to your weapon's specific table, indexing by your modified roll vs. his armor type to see the effect. This is largely building the game around the concept (much as the Chainmail miniature rules which predated D&D did). Rolemaster also gave each monster an equivalent armor type and included tables for various natural weapons. Back in the 80s I recall a lot of D&D groups used the Rolemaster Arms Law/Claw Law books for their D&D games - and the Rolemaster books encouraged such usage.
As far as speed factor goes... To this day I don't fully understand precisely how it was intended to be used in AD&D 1st edition. From some Googling it seems that its intent was in cases where initiative was a tie (which was reasonably likely given the use of 1d6 for initiative in 1st edition) speed factor would both determine who acted first and it allowed multiple attacks in a round (which was a minute long in AD&D 1st and 2nd editions). This is another area where I think AD&D 2nd edition improved things. There, as in 1st edition, you rolled initiative every round. However in 2nd edition you used a d10 and the speed factor was added to your initiative (making low rolls better). It added some complexity but was an optional rule that I usually ended up using.
Overall, I prefer AD&D 1st edition to its 2nd but looking back on it I can definitely see some areas where 2nd edition made much-needed improvements to the rules. Of course they also took away demons, devils, daemons from its monster list and removed assassins and half-orcs from the Players Handbook... (Boo...)