Especially with the advent of later editions of the D&D game, as well as the influence of video games, recent fantasy films and other factors, many players will wish to play Cleric, Magic User or Thief characters that break the standard Classic D&D model of those archetypes. Sword wielding Clerics, Magic Users adventuring in armor and other odd combinations are likely requests from players creating new PCs for a campaign. The first reaction from many a DM, including me, is to say no and demand everyone play by the rules as written, however, I've slowly come to the conclusion that the players should be able to create the character they want, within reason, and the rules can be slightly tweaked to allow these weird PCs.
Fighters, Dwarves and Halflings are allowed (almost) any weapons and armor. The only exception to the rule is the use of very large weapons by halfling PCs. This should just be a matter of common sense, I won't waste time combing through the weapon lists in detail, but suffice to say that from the Basic rulebook weapon list, the Battle Axe, Long Bow, Two Handed Sword, and Pole Arm should be barred to Halflings. At the DM's discretion, a very strong (Str 16+) Halfling PC in a life or death desperate situation could possibly wield one of these once in a while using an "untrained" penalty (explained below) of -1 to hit and -1 to damage. I recommend discussing this possibility in advance with the DM and other players, to avoid arguments within the game.
Thieves are well rounded combatants, skilled in the use of many weapons and armor types. The restrictions placed upon the class tend to reflect a preference for light, quiet weapons and quiet, nonrestrictive armors that do not hinder any of the class abilities. In the event that the player wishes his thief to wield a weapon not normally allowed to the class, I impose an "untrained" penalty of -2 to hit and -2 to damage. STR bonus or penalty to damage is still applied in addition to these modifiers.
In the unlikely event that a thief character wishes to don heavy armor, I would allow it, but with some dire consequences. First, the character operates with a -4 penalty to DEX, and is unable to use any of his thief abilities while wearing any outlawed armor.
Clerics are the least restrictive class in combat, outside of the Fighter group. With a choice of any armor type, the only request you're likely to encounter is the use of a barred weapon.
In a campaign that uses a specific Mythos or Pantheon of gods, each with a detailed portfolio, mythology and iconic weapon used by that deity, I allow the cleric a choice, to be made at character generation and unable to be changed later without 1d6 months of "offstage" training time and the loss of one level of experience. The PC may either use the normal cleric selection of any blunt weapon, or forgo those weapons and be trained in only the specific weapon of his deity.
For Example: Artemy, a Cleric of the god Ares, might decide to forgo the use of the normal selection of blunt cleric weapons, allowing him to train in the use of the Gladius (short sword), the favored weapon of that god.
In the event of a god like Ares or Athena, where two or more weapons (short sword and/or spear, in these cases) might be considered iconic, the cleric must still choose only one of those weapons to be trained in. A Cleric who uses a weapon forbidden to him suffers the same untrained penalties a thief does; -2 "to hit" and damage, and must additionally make a successful WIS ability check each time the forbidden weapon is used or face the wrath of his god and lose all spellcasting and undead turning ability for 24 hours. Desperate, life or death situations may warrant a modifier to the WIS check, at the DM's discretion, and repeated willful violations may invoke harsher punishments, again, at the DM's discretion after a proper omen or warning is given by the cleric's god.
Magic Users suffer the harshest restrictions on the weapons and armor they may use, as well as the toughest penalties when violating those restrictions.
In the event that a magic user wishes to don armor, I would allow it, but with some dire consequences. First, the character operates with a -2 penalty to DEX, and is unable to cast any spells while wearing any armor.
Elves are somewhat trained to use magic while wearing armor and do not suffer a DEX penalty for wearing armor, but each time they cast a spell while armored, they must make an INT ability check to successfully complete their spell. A failed check means the spell is interrupted and lost from memory.
When attempting to use a weapon barred to their class, Magic Users suffer the usual untrained penalty of -3 to hit and -3 to damage, coupled with their likely penalty to damage from a low str score, if applicable.
The other thing to keep in mind when characters, such as Magic Users or Thieves, with low STR scores attempt to equip bulky armor and heavy weapons is the impact on encumbrance. Even in campaigns where carrying capacity is handwaved, I suggest a DM use common sense and not allow situations where a STR 6 Magic User is toting around (though perhaps not actually wearing...) a suit of plate armor and a couple long swords, in addition to his spellbooks and other adventuring gear.
Any thoughts? Feedback is welcome!