This are my house-rules so far. Some rules reflect the necessities of the campaign and its participants (time constrained young-adults). These houserules are not engraved in stone!
BOOKS USED: PHB is the main player book; UA only used for some spells (negative plane protection and remove paralysis most notably up till now); D&DG for stats over 18; MM1, MM2 and FF monsters used, many monsters made by myself; DMG is regarded as a sourcebook, not a rulebook.
ABILITY SCORE GENERATION: 4d6 drop lowest, 6 times, then worst result becomes a 18. Poor stats (DM’s call) are re-rolled if player wishes so (some players enjoy playing crappy characters!)
CLASS/RACE: PHB restrictions in full force, but I’ve been a bit permissive with level limits. If it fits a character, I will slightly modify choosen class specially for it, to make the character more unique (up till now, modified the Fighter slightly into an "Archer", allowing that character to split-move and fire, but losing extra attacks due to high level with melee weapons). Thief class discouraged! (this is the Whitebox OD&D guy in me).
INITIATIVE: is group 1d6, higher goes first, ties mean simultaneous action. Spells must be specifically declared before initiative is rolled. Other actions are declared more generally (such as "I attack"), and the specifics are determined during your turn. I also ad-hoc rule some situations that go outside from that roll. For example, bows shoot before charging opponents, pole arms attack first in first round, etc. This means I ignore speed factors, segments and casting times, except for spells that need a round or more to be casted. Spellcasters that lose initiative and are hit, lose their spell. Power-Words cannot be interrupted, nor do "at-will" spell-like abilities.
Suprise is 1-2 in d6. 1 in 6 if DEX 15+. Lights and sound due to heavy armor make it almost impossible to surprise enemies.
OTHER COMBAT: Natural 20s do MAX damage. Natural 1s always miss. Situational bonuses or penalties are ad-hoc ruled. 0 hp unconcious, -1/-9 bleeding 1 hp per round, -10 dead. When you fail a save vs. poison, you have 1 more round of action (hopefully a spell can be cast on you or you can drink a potion), then you die. Survival shock is not rolled if you are "Hasted" (you still age though). Len Lakofka's To Hit tables.
WEAPONS: I ignore weapon proficiencies (you may use with no penalty all weapons allowed to your class). I don't use different damage for large opponents nor weapon vs. armor. I make up new weapons using the one in the PHB as a guide (for example, the Paladin player wanted to use a Maul, so I ruled it's a 1d10 two-handed bludgeoning weapon. We don't keep track of normal arrows, I just require players to spend some money each time at town for an abstract "replenish".
MAGIC: spell components are for flavour, except expensive ones with a specified price. Some spells are as per AD&D2E (single arrow mode of the "fire arrow" spell, the only one applied till now). Cure spells cure a minimum of half cleric level, but no more that max possible of the spell.
ADVENTURING: Encumbrance is eye-balled. We don't keep track of food or water (except in dire/obvious situations, such as being trapped in a cave or something of the sort. In such situations I assume the party has a week or so left of provisions). Ability Score checks (d20 roll under stat) used now and then for climbing, jumping, etc. Character knowleged is based primarly on class, but all characters are considered to have the ample skills of a competent adventurer. No secondary skills or NWP used. Movement slow-down due to armor mercilessly enforced! (if you are wearing plate mail, you'll have a hard time running away!). Sleeping recovers 2 hp per level.
ALINGNMENT: is meant for "color" and to help quickly define a broad ethos/world-view of the character. It is not strictly controlled and is interpreted with a lot of flexibility as to not stall down the game with ethical arguments. No alignment languages. Heck, nearly everyone in the world speaks common!
LEVELING: Experience is not tracked, but arbitrarily granted by the DM at the end of each "adventure". No training needed. All HD is re-rolled and you keep the result if higher that previous HP total. If not, you just gain +1 hp (an adaptation of Empire of the Petal Throne).