I don't know how many people in the OSR have ever participated in a LARP, let alone a combat intensive one. In the early 90's I was heavily involved in "padded weapon" LARPs in the DC area. It was a fun activity, and at times provided some cool stories and excellent roleplaying opportunities, not to mention good exercise. After a few years I got out of it because I had other interests, and was generally tired of the political/personality conflicts in these local groups. But in 4 or 5 years I did discover several things that I can apply to tabletop games as well. Most of what I learned was about social interaction and roleplaying, but some of it can actually be applied to tabletop rules. WARNING: I'm not trying for "silver age" realism here. I am trying to put some "real world" experience to good use. I don't want to add overly complex systems to the game, put I do want to examine existing systems and rules in light of my experience on the padded weapons battlefield.
The first subject might not be that obvious, but I can not emphasize enough the importance of morale. Creatures of any sort of intelligence are not going to get themselves involved in a losing battle, except in extreme circumstances. Let's take a hypothetical. An adventuring party of 6 kicks in a dungeon door. In the room is 4 goblins. Typically you roll for surprise and initiative and then combat begins. If the party attacks first, the goblins will engage in combat. In B/X rules, morale is first checked when the first death on that side occurs. Under 1e, it's checked when the leader is defeated, 25% of the force is eliminated, or when they are facing an obviously superior force ( which is left up to the GM's discretion). From my experience in LARPing, people think twice before engaging in combat (even when it is not life threatening), and only pick fights they think they can win. An outnumbered group is going to retreat or surrender unless those options have been taken away from them, or they are absolutely sure that their opponents are inferior. They also tend to steer clear of the best fighter on the other side, preferring to take out the weaker targets first to get the numbers in their favor. In the above example, the goblins are not going to fight if they see a group of well armed men kicking in their door. On the other hand if they were to see 6 peasants stumble in, the goblins would probably be up for a fight. Of course, this all depends how goblins view themselves in your campaign. Maybe they are chesty little buggers that are always biting off more than they can chew, or maybe they are bullies that quickly turn coward as soon as they are pressed. But no matter what, morale or reaction needs to be considered at the start of the encounter. The takeaway from my experience is that using 1 number to represent a morale rating, or a random roll to determine reaction is probably too simplistic. It's much better for the GM to take the time to think about the psychology and morale of the monsters the players will be encountering, and figure out how they will react to different situations. Less rules and roles, more thinking.
Missile weapons and combat are a dicey thing. It's damn hard to hit a moving target, and anyone with a shield is incredibly hard to hit if they are watching you. Most of the time firing into a melee is pure chance. An archer attacked in melee is screwed. Your best bet as am archer is to shoot at stationary targets that are not paying attention to you, and stay as far away from the melee as possible. This often means targeting other missile weapon users and casters. There are 2 things that I would change in the rules in regards to archery in particular. The first is that shields should count for more than a simple +1 or +2 when attacked from the front. The second is that crossbows are much more simple to use than others bows. It takes 5 minutes to learn how to reload, and then it's point and shoot. I longbow takes a lot of practice before the user can shoot straight on a consistent basis, and would require a decent amount of strength to be effective in combat. I'm still not sure how to represent this in the rules. 3e places them in the simple weapon category, which allows for a wider range of classes to use them. While this makes logical sense, I really don't like the idea of wizards and clerics walking around with crossbows. Thieves would be ok though. The best I can come up with currently is a simple +1 to hit across the board for crossbows. Minimum strength requirements for using regular bows is an option, and could come into play if you use 3d6 in order to generate characters.
I mentioned shields while discussing missile weapons, and this is probably the one area where my experience LARPing changed my thoughts on tabletop rules. Shields are tremendously undervalued in D&D. This difference in attacking a person with and without a shield are night and day. Since 3e was published, I have used the rules for shields found there for all of my games (large shields are a +2, tower shields +4, can provide cover against missile weapons), but even that is not enough in my estimation. However, the one thing I can't judge from LARPing, is how much protection armor provides (that's a whole other debate). Any change in AC bonuses from shields needs to be judged against the AC bonuses that armor provides, and I am hesitant to make shields too powerful, if only for game mechanics. A +2 for small, +3 for medium, +4 for large (tower) system seems to be a good compromise. I haven't included bucklers in the discussion because I think they are special case. I see them more as a specialized tool for parrying, rather than a static defense in the way that armor and shields are considered. Also, as they more properly belong to the renaissance period, I wouldn't use them in my games unless I was doing something in the swashbuckler genre.
Historically, shields are generally made of wood, with metal and leather used to hold it together. As you can imagine, when hit with a sharp heavy weapon swung at high velocity they sometimes break. This is something we simulated in the LARP rules that we used (3 hits to a shield from a polearm destroyed it), but we also encountered actual breakage occurred with shields and weapons. RPG rules don't normally account for weapons shields and armor breaking, unless some sort of critical fumble rules are used. I haven't figure out a simple way to account for breakage yet, as I feel that it probably requires the type of rule that adds more complexity or book keeping.
Something that I learned from LARPing that is often touted by MMA practitioners is that fights often end up on the ground. That isn't to say that grappling becomes a major part of combat, but that in reality people fall down more often than is typically portrayed in tabletop RPG combat. Some editions of the rules have abilities that can cause a a character to trip of fall over, but what I noticed is that people have a tendency to trip over themselves or lose their footing when the ground is uneven. As a GM, you might want to think about what type of surface the combat is occurring on. Something as simple as a forest or a field can be treacherous footing, Consider the dangers of gopher holes, loose rocks, and tree roots sticking above ground.
Closer attention should be paid to the terrain in general when in outdoor environments. For example, small trees, bushes, and other undergrowth can be a real hindrance to movement in a forest, without blocking line of sight for spells and missiles. In my experience, archers and casters had a huge advantage in the forest environment because of this. Larger shields, polearms and packs easily become entangle or blocked by bushes and low hanging branches. Again, this is the sort of thing that should be the GM's call. Not all forests are the same, and neither are all swamps, rivers, hills, grasslands etc.
This post went on a lot longer than expected, but as I was writing, more things kept coming to mind. There's probably more I could come up with, and if I do, I'll post more.