Rules from my Pacificon OD&D game

With just the three little brown books, thieves were not an officially supported class with trap disarm and backstab tables etc. Had someone wanted to run a thief, I was prepared to invoke the "even a Balrog" rule to cobble together a simplified thieflike class on the spot, or shade a fighting man who goes light on the armor to have some on the fly advantages for it that would be thief-like. Notably, I wanted to avoid the percentile thief skill tables, and wanted trap evasion or disarming to be the old school figure it out what you are doing and maybe roll a D6. While I announced and described the rule, it did not come up directly as nobody really opted for anything too far outside the box.

Among the houserules in my long 70s high school D&D campaign (after my first megadungeon game imploded on bad DM calls) I developed an elaborate spell point system. One of its features was to make utility spells generally cheaper than combat spells, so a beginning mage could take a couple Light spells or Read Magic and still cast Sleep, if maybe a bit weaker on the number of orcs it would knock over. It felt too "advanced" and too cumbersome to use for this one-off though, so I went with the simple expedient of saying mages could memorize one 1st level combat spell and three utility spells, and told them which counted as combat spells (just sleep and charm person on the original short first level list). This worked pretty well, giving mages some fun magic to do without wiping too many encounters with Sleep. Not exactly sure how I would scale it up with levelling, but it might be worth some effort to knock together a variant that has the same property of simply adding more utility magic while keeping combat spells on the strict Vancian memorization limits. 

Since an original cleric has no spells at first level, I gave the cleric a couple scrolls of cure light wounds as backing from his temple superiors for his mission.

Hit points I did with the original all D6s table, but I had any roll below 4 on the first hit die be a 4. So no two HP PC fighters or 1 HP clerics or mages. Monsters did not get this bump, so a goblin might be a 1 HP wonder.

Stats I had them roll 3D6 for each statistic, no swapping, but let them bump one up to a 14 if they had no rolls of 14 or higher, which would both give some opportunity to customize towards a class and make up for poor rolls. If they were notably bad, I gave one re-roll of the whole set of stats. Only one guy ended up using that chance and he ended up keeping the original set of rolls since the rerolls were worse. So this is almost as tough as the original system.

Damage was 1D6 for normal hits by any weapon, with a natural 12 to hit being a max damage crit. But the Chainmail man to man 2D6 to hit tables give some widely varying chances to hit based on weapon/armor interactions, with two handed swords being awesome pretty much across the board, and daggers and short bows being quite good against lightly armored targets but poor against heavy armor, so fighting men had some serious advantages in combat with the full range of weapons available, but magic users are surprisingly good against very lightly armored targets like the mook goblins they faced most often in the castle ruins and first dungeon level. In general weapons that would do more damage in later D&D tend to hit more often on this table so get a net effect in the same direction.

Stat bonuses were a bit more liberal than the original but less than Greyhawk. STR 13-15 got +1 to hit, 16 - 18 got +1 to hit and +1 to damage, for instance. Also gave high strength characters a +1 on the door opening checks. These bonuses are bigger than they seem, given the 2D6 combat bell curve and 1D6 universal weapons damage.

I ruled the one inch space on the floor grid was 3.3 feet instead of the more typical 5 feet in current rules, so we could have the classic close order 3 man wide in a ten foot passage formation, fighting a couple ranks deep if the second rank had appropriate pole weapons or a short first rank. I was planning to penalize greatswords that didn't give themselves lateral swinging space, but didn't end up enforcing that, might do it if I was to run with Chainmail combat tables long run.

I played the sequencing pretty much like Chainmail man to man with very long weapons getting first swing advantage on closing, and short weapons getting first swing and potentially extra swings at the end of the round if the length difference was really big on subsequent rounds, with initiative on a D6 deciding move order and and swing order in cases where the lengths were close. So everyone moved and/or did ranged attacks by initiative order, then all melee attacks took place by hybrid length/initiative order. Magic did not get high enough in level or frequency this time to make me deal with whether higher level spells took longer, could be interrupted, spells were slower to cast from scrolls, etc.

Cinematic declarations were either handled in colorful descriptions of the results of standard rolls, or with off the cuff D6 or stat based rolls to succeed if trying to achieve some affect outside the ruled bounds like a knockdown or disarm or a tricky move like surfing down the stairs on a broken door. Yes, there were a couple of characters doing multiple wipeouts on the winding stairs down to the second level at the end as they kept failing to emulate Legolas's shield skateboard prowess.