Lately I've been running a very "old school" style of campaign wherein each session the characters enter and return from the Barrowmaze, a mega dungeon written by David Gillespie. I assembled the rules myself.
In recent years D&D players have been posting their house rules on the internet. Many have published their versions of D&D as “retro-clones”, games unto themselves. Among the most popular are Labyrinth Lord, Swords & Wizardry, Lamentations of the Flame Princess and Dungeon Crawl Classics, but there are now more than 300 various pdfs and books floating around, each offering some variation to taste of the d20 Dungeon's and Dragons experience. At some point the obsessive D&D fan, like a connoisseur of fine wine, must mix and match and blend his own vintage. Hence, Uncommon Dungeons.
My priorities with these rules were creating a very fast playing game that emphasizes exploration and emergent play over customization and tactical combat.
To minimize math at the table I decided to vary the type of dice rolled instead of applying modifiers (+’s and -’s). This brings about the least standard element, which is the use of the Dice Chain, adding d3s, d5’s, d7’s, d14’s, d16s, d24s and d30s to the standard polyhedral set. This can make for some searching at the table for the right die, and some searching the 'nets in order to purchase those dice in the first place, but weird dice are fun and they allow for the roll under mechanic. As we have been playing online via Google Hangouts, my friends have been using dice rolling apps.
A priority is to make direct use of the 3-18 ability scores. In most D&D games, scores are generated rolling a handful of d6s, those numbers are then converted to +/-4 modifiers and the scores themselves are hardly used again. This has bothered me for well nigh 25 years. When I played Basic D&D, long ago in the middle school days, “skill checks” were made by rolling under a score with a d20. This made use of the score, but it created a situation where-in a high score ability would nearly always guarantee success and there was no room to grow. Using d30s and d24s roll under mechanic for skills and saving throws makes success far less likely but allows a character to improve with time.
The combat rules also de-emphasize modifiers. The strength score does not improve chances to hit in melee, while dexterity does not increase damage with ranged attacks.
The extreme danger of arrows and spears is modeled with an “exploding dice” mechanic.
At the same time I have been enforcing stringent penalties for launching missiles into melee combat, the ease of which is a pet peeve of mine in D&D V.
Although simplification is a priority, I like spellcasters to have a bit of unpredictability and danger, so there are rules for risking blood and sanity to perform additional and more powerful magic. On the other hand, I have no interest in writing new spells, so the spell lists from from a variety of sources are available, mostly B/X and LotfP.
Finally, there are a number of niggly combat rules to encourage a quick and dangerous style of play based on choices rather than character options. This begins with weapons and armor.
Shields were historically the most crucial armor, so the defense bonus is higher than is usual, a +3! Conversely, two-handed weapons do quite a bit more damage, so the trade-off is a decision based on desired fighting style. Similarly, ranged weapons can be very effective, but become much less reliable when hand to hand battle is joined.
There is a balance between maintaining the possibility of serious consequences and over-doing it. For example, a character that is poisoned makes a Constitution saving throw to avoid death, a terrible consequence mitigated a bit by time. The player is helpless and dying for d100 minutes. If players can find an antidote in time, the poison may be counter acted. Similarly, the touch of many undead monsters is often a death sentence. In these rules, the black touch is tied to fear and sanity as represented by the Wisdom Score. Characters lose their grip on reality as they delve into the unknown. This becomes another way to lose a character and makes the undead especially fearsome without level drain or extremely temporary hit point reduction.
In any case, I've been enjoying the home brew ruleset to go with the prefab dungeon module.