Thoughts on equipment restrictions

 Weapon and armor restrictions by class are some of the most iconic features of D&D (particularly older editions. I'm not well-versed in 3e and beyond.) There are a few reasons for it, primarily game balance, but also in-fiction flavor, to depict a particular archetype in a class.

Weapon restrictions are consequential in a few ways. In editions with variable weapon damage (or with optional rules for such in use) they limit the amount of damage a character can do in battle. A magic-user limited to dagger only is also limited to d4 damage, where other classes have access to d6, d8, or even d10 weapons. A cleric's restriction to non-edged weapons effectively eliminates the d10 and d8 weapons (those dice being mainly used for swords, polearms, and battle axes) and the d6 missile weapons, leaving only the d4 sling. Even if you're playing d6 damage for all weapons, weapon restrictions still have one important effect, as many others have already noted: it sharply limits who may use "found" arms, including most magical weapons. A cleric can easily choose to buy a mace instead of a sword, since generally character creation allows carte blanche selection from the equipment lists, but if you've found a magical sword, you can't very well choose for it to be a mace instead. Armor restrictions are likewise tools of game balance, since to a great extent they determine survivability in combat. 

Setting aside the game-based "why" of weapon and armor restrictions, the in-fiction "why" is at least as interesting to me. The easiest explanation, of course, is that it's just a game, and equipment restrictions are just features of that game. That's all well and good but it does grate against the popular old school ethos of being able to try anything. There are also some perfectly valid reasons why a character might need to pick up a sword or an axe and a flat, "You can't do that," doesn't answer very well. Also, I'm not aware of any weapon and armor restrictions on zero-level "Normal Humans." Basic D&D certainly doesn't have any. I distinctly recall "Normal Man" NPCs in The Keep on the Borderlands who use various weapons and armor. The idea that a character taking up an adventuring class loses the ability to use weapons or armor is more than a little nutty to my mind. The stock rationale for such restrictions is that some classes aren't trained in the use of every weapon or armor, but this implies only that those classes aren't very good at using those weapons, not that they're physically unable to pick one up and hurt someone with it.

If we assume that some classes just aren't good with certain equipment, we're left with a question which is rarely asked and even more rarely answered: What happens when a character tries to use a prohibited weapon or armor?

For weapons, one possibility is that characters just never get any better with proscribed arms. A normal human can use them, just like he can use all weapons, at normal human level of ability -- in classic D&D, that means he gets no bonus to attack, or a THAC0 of 20. A character trying to use a weapon not normally available to his class attacks as a normal human with it, whether he's 1st level, 5th level, or 14th level. (1st level classed characters in B/X and BECMI all have a +1, or a THAC0 of 19.) 

This does pose a potential problem, especially for low-level characters, and most especially for low-level magic-users: Why not use a sword or a polearm for more damage, especially when they don't get any better at their attack rolls until 5th level anyway? It's basically a -1 penalty in exchange for a sizeable damage boost. The normal human attack proficiency could be paired with a damage penalty of one step (e.g. a d6 becomes d4, d8 becomes d6, and so on) which you would then apply to zero-level characters too, just to keep it fair. Perhaps d4 weapons are usable by everyone with no damage penalty, just because they're usually the easy ones that require little expertise, like clubs and daggers. I can't see any harm in letting magic-users use clubs, can you? If anything, clubs should be preferred even over daggers by M-Us for sheer ease of use!

There are potentially other reasons a class can't (or shouldn't) use certain weapons, which I'll distinguish from the above by calling them cultural reasons, rather than purely practical. Most famous of these is the cleric. A cleric character chooses, in-game, to give up edged weapons because his or her faith actually forbids it. It's based on a historically inaccurate portrayal of medieval Christianity, but there's no special reason why a fantasy religion couldn't impose such restrictions (or others) on its clerics. (Why clerics of every religion and cult, especially evil ones, should be identically restricted to blunt weapons is another matter entirely. It's perhaps a good case for ditching the cleric as a class per se and leaving religious leadership as a cultural role only, but I digress.) Obviously a cleric who chooses for whatever reason to use a sword or dagger should suffer the "untrained" penalties above, but also might be afflicted with other penalties for violating tenets of faith. Maybe their spell-casting and undead-turning abilities are hindered or lost altogether, or they suffer a curse of some sort, at least until they perform some act of contrition and atonement. Maybe they're just excommunicated or otherwise sanctioned by the church if word gets back to the higher-ranking clergy.

Armor is a thornier issue, but not insurmountable. Armor requires training to wear comfortably, too. The most obvious effect of wearing armor to which one isn't accustomed is extra encumbrance: the character moves as if one level more encumbered than he actually is. Of course this should apply to normal humans in armor too. Metal armor should make such thiefly activities as stealth, listening for faint noises, climbing walls, and performing delicate operations on fiddly mechanisms like locks and traps very difficult. I would say they should function at one-third-level or (in the case of listening and chances to surprise) at the same chances as non-thieves, whichever is greater. Magic-users could be penalized similarly, casting at half-level or with a chance of failure when armored due to restricted range of motion and general discomfort. It would be tempting for thieves and magic-users to slip into armor when they're not sneaking or casting, but remember, armor can't just be donned and doffed at will. It takes a bit of doing, at least a turn, and they'd still have to lug it about and suffer its encumbrance even when not wearing it. Seems like a bad deal to me.

A few restrictions make sense in an absolute or near-absolute sense, generally those relating to size. The idea of halflings using longbows is just absurd, though there's no logical reason why they couldn't have scaled-down polearms doing a die smaller damage than their human-sized equivalents.  

It gets really odd with shields, though. Normal humans can use shields. Why can't a magic-user or thief? A large shield might be a genuine hindrance to thief skills, but surely a little hide buckler or something similar isn't out of the realm of possibility. For magic-users, it's reasonable that they'd need at least one hand free for spell-casting, so maybe they could hold a shield instead of a weapon, but could it be used effectively for defense while casting? He can't use his weapon to attack while casting, after all. If he's not casting, or if he's used up his spell allotment for the day, he might use a shield with his dagger or club, but even with a shield, wading into combat is a sucker's prospect for a magic-user. Ah well. In any event, a simple +1 to AC probably isn't worth quibbling over if a thief or MU wants to strap a shield to his forearm. 

And that's all I have to say about that.