Remembering way back to the piles of house rules we had. . .
No Dual-Classing, but Humans can Multiclass like any other race.
Any multiclass combination is allowed as long as alignment restrictions don't contradict (but can still only have 3 classes max)
Any multiclassed character may elect at any time to stop progressing in a character class and no longer spend XP on the class, but it takes a Limited Wish or Wish to return to progressing in that class.
Most religions had a custom-built Specialty Priest class, the generic Cleric class was strongly discouraged and only used for religions that didn't have a specialty priesthood or another priest class that was more appropriate. The Crusader, Mystic and Monk (2e version) classes from Spells & Magic and Faiths & Avatars were used.
Druids didn't have to duel for rank, and the whole idea that all druids were part of one giant worldwide order reporting to the lone 15th level Druid was thrown right out: druid was just another priest class along with Cleric, Crusader, Mystic, Monk, and Specialty Priest.
We had a homebrew "Psionic Warrior" class that was very similar to the Psychic Warrior that later came out in 3e.
We had a NWP called "Self Improvement" that you couldn't start with (You had to find somebody to teach it to you in-game) that let you slowly raise your prime requisites (using a rather complicated system of percentile dice), the 3e "one point every 4 levels" was a vastly simplified version of this that everybody preferred.
Every Fighter got exceptional strength even if they didn't have an 18 (we figured out how far above a base 18 every exceptional strength category was, you rolled for your exceptional strength, and got that much of a bonus for your exceptional strength on top of your normal strength).
1e Assassin and 1e OA Samurai were also back in.
Demihuman level limits were mostly ignored, technically they still were there but if you had a 16 or higher in the Prime Requisites you got to ignore them totally, so in practice they were ignored.
A Natural 1 was a fumble, then you had to roll percentile dice to see what kind of fumble it was with a table of results, low percentile rolls meant you injured yourself, high ones meant you injured your allies, mid-range ones were merely missing, and extremely high or low ones meant you re-rolled and results stacked (in one extreme case, someone rolled very low several times and very high several times before getting a middle-of-the-road "miss" result nearly TPKing their entire party with a catastrophic fumble)
A Natural 20 was an automatic hit for maximum damage, unless you were fighting a God (yes, that came up on very rare occasion) whereas the true form of a deity could only be hit on a Natural 20 unless you were using an artifact-scale weapon, and even then unless it's an artifact the attack automatically did minimum damage.
Every character could roll at character creation for a psionic wild talent without the risk of mental injury. The chance of injury only happened if a character tried again to gain a wild talent after character creation via psychic surgery. (Psionics played a big role in all our games)
Spell Points from Spells & Magic were used instead of spell slots.
All spellcasters had a number of Cantrips/Orisons they could cast per day equal to their character level in addition to their normal spellcasting progression.
We had a fairly extensive house-rule system for spell research, where libraries of varying complexity let you research spells of various levels in various schools, and costs and times varied on if you had the Research and/or Papermaking NWP's.
Spell research was an integral part of playing a wizard. You only got a new spell automatically when you got a new level of spells to cast, so after character creation you would only have 1 2nd level, 1 3rd level ect. spell unless all the wizards in the party shared spells (which was typical, a wizard that didn't open his books was looked upon very suspiciously), spellbooks as loot were incredibly rare, and researching new spells (and discussing/debating spell details with the DM) was a very big part of playing a wizard.
There was an increasing hierarchy of Weapon Mastery rules for Fighters. After getting Weapon Specialization, they could continue to get more and more bonuses with their favorite weapon by putting more character points into it (we used the Skills & Powers system for proficiencies), High Mastery, Grand Master, and Supreme Grand Mastery were available.
In our Forgotten Realms campaigns we had one homebrew deity. Aliah, the Demigoddess of Invocation. She was to invokers what Velsharoon was to Necromancers and Savras was to Diviners, a patron demigod of a school of magic. She was Lawful Good and was originally a fire-oriented invoker from Calimshan (actually the PC of one of the players from a campaign in the early 90's, and many years later ran a plot as DM where she ascended, and the rest of the group thought it fit so well we used her as part of the Faerunian pantheon whenever we ran the Realms). When we converted to 3e we made her favored weapon the scimitar and she got the domains of Law, Good, Fire, Magic and Knowledge
