For whatever reason, I will try to put together some house rules and issues from back in the day, in no particular order:
1) Clerics and Priests: really three things--clerics know too many spells, specialty priests seem like a cool idea, and clerics have utility spells that can impact play, if only they were prepared in the first place. My solution, limit the number of--spheres, I think--but allow what would later be called spontaneous casting. Also allow druids to have spontaneous casting. Also, for specialty priest, don't be afraid to give them a few cool things on top of their spells. (yet another alternative is to allow all casters spontaneous casting, but not be able to cast the same spell more then 1 time per day. yet yet another is to do a ritual type thing, and cast from a spell book or through prayer at an increased casting cost in time or gold).
2) Running out of spells: long before at will magic, we worried about this. In hindsight, AD&D addresses this through wands and other items. Which is a good solution, you can increase flexibility quite a bit, keep things under (DM) control, but there is still a trade off in charges. If this is not enough, you can maybe allow some kind of over casting, ie cast an unprepared spell and lose 1d4 con for a day.
3) whimpy thieves and skill users: some people love the thief archetype, otherwise no one would have bothered to play one. I suggest giving a boost on % chance of success for easier (ie low level) tasks of at least 20%. Rangers and other % skill users can also benefit from this. Also, be generous with backstab. Let it happen. (also see 10 below)
4) multi and dual classing: let non-humans dual class and humans multi class. Its fine.
5) non-human level limits: We used an XP penalty (double I think) instead and I still had plenty of non-humans. What I might do know is let humans add +1 to any ability (up to 18) and maybe give a 10% XP bonus, and again remove the limits.
6) They die so fast: in the past, max HP at level 1, fudging some roles, and a generous death door policy. Though just giving +10 HP would be simpler, but maybe no purist enough. For save or die, maybe interpret die as "0 hp" (but still allow save or stone, charmed, etc, etc)
7) THACO...ugh. Use those sheets were they fill in a little thing were they see what AC they hit...
8) Fighting fun: weapon specialization is good, use it, and maybe allow others besides fighters to give up WP slots for it (not sure what the RAW was...maybe fighters the only ones at level 1?). We tried various maneuver rules. I might allow a called shot rule (-4 to hit to do something interesting) if it was clearly not being abused.
9) kits: I liked them, I hated them...but some of them were alright. This is were the OGL/OSR crowd could do something useful, not sure they have.
10) really pushing it...replace roll under NWP with a % based skill system. Maybe use some existing NWPs and skill or skill like things and then have a standard set of %s based on ability score and or level.
Here is a list of the optional rules and house rules we have decided to start with.
Attacks of Opportunity
You can make one AO per creature, max 1+1/5 lvls (warriors 3+1/5) per round.
Training
When you level up, you automatically gain whatever you already have. If you wish to gain new abilities, such as new proficiencies, new spells, etc, you need to seek out an appropriate trainer - and pay the cost.
Movement
Diagonal movement costs 50%
You can withdraw (and avoid AO's) from combat by doing nothing but moving during your turn and moving 1/3 of your movement.
Facing
There is facing. The facing of a creature can only be determined during it's turn
Initiative
We use the optional rule of individual initiative, including weapon speeds etc.
Hit Points
Whenever a character gains a level (up to level 9 or 10), he rolls as usual or gets half rounded down, whichever is greater. He then adds his CON modifier. Fx: A fighter rolls a 2, but since half rounded down is 5, he gets 5 hit points + CON modifier.
Healing
Magical Healing starts at 0 hit points
Natural Healing starts at current hit points (can be negative).
Natural healing is 1 die per odd level of class.
A 1st level fighter heals 1d10 hit points per night's
A 5th level rogue heals 5d6 hit points.
A successful herbalism check adds another 2 Hit Dice worth of healing.
If you are reduced to 0 hit points, you are unconscious
If you are reduced to negative hit points, you lose 1 hit point per round on your turn. If you reach -10 hit points, you are dead.
Proficiencies
If you want to take proficencies normally not available to your class, the NWP costs an additional slot.
You can use most NWP's even if you do not have it. But your stat is halved.
You get a number of bonus non-weapon proficiencies based on your intelligence (equal to number of languages). If you are a warrior (fighter, paladin, ranger), you may use those bonus proficiencies to acquire additional weapon proficiencies.
It is possible for fighters to gain weapon mastery (+3/+3) with a weapon. Doing so costs 3 weapon Proficiencies and can not be acquired at level 1
There are five weapon styles that can also be acquired by spending weapon proficencies.
Weapon and Shield Style: Gain ability to make shield-rush, shield-punch, block and trap without losing the bonus to the shield.
One-handed weapon style: +1 AC bonus when fighting with a one-handed weapon and no shield or offhand-weapon. Investing 2 WP's causes the bonus to rise to +2.
Two-handed weapon style: increase speed of weapon by 3. If using a one-handed weapon in two hands, you still get the increased speed of weapon, but also a +1 damage bonus.
Two-weapon style: Penalties go from -2/-4 to 0/-2. If an additional slot is used, you gain the ability to use two weapons of equal size.
Missile or thrown Weapon Style: Ability to move up to 1/2 movement and still have full rate of fire or make full movement and half rate of fire. They also gain a bonus of -1 AC against missile fire when attacking with a ranged weapon.
Racials
There are no racial minimums.
There are no racial level limits
There are no racial limitations on class
To balance humans with the other races, they get a +10% xp bonus, +1 hp/level
Attacks
Critical hits = Max damage + normal damage, so if a character normal deals 1d8+6 damage, he deals 14+1d8 damage on a critical hit.
Firing into melee: Unless the target has cover, there are no penalties. If cover, then -2 to hit.
Called shots are in (with a few modifications)
Classes
Anyone can multi-class
There is no dualclassing
Both specialty priests and clerics use spontaneous casting
It is possible to cast a known spell as ritual. The cost is 1 ritual stone per spell-level
Thieves get backstab bonus when attacking from the rear.
