I am currently using the Realm as a campaign setting for one Labyrinth Lord campaign, and plan to use it for my second, Advanced Edition Companion campaign once I can get that started.
Inspiration for the Realm
As you can tell from its history, the Realm is a post-apocalyptic fantasy setting, though it is still quite strongly High Medieval/late Renaissance in culture and style. Science-fantasy elements are minimal, though present.
The strongest broad influences on the Realm are the default assumptions present in the game itself... there are a lot of dungeons filled with treasure, there are monsters crawling all over each other in the wilderness between the small settlements, and there are a lot of adventurers seeking to loot said treasures and slay (or better, avoid) said monsters.
The strongest pre-existing world that has the strongest influence on the Realm is the Realm of Dungeons & Dragons presented in the old Dungeons & Dragons cartoon. Though I've scrapped the "Dungeon Master" and "Venger" characters and duality, the rest of the nature of the world and its design assumptions are the strongest influence, stylistically, for the Realm.
The works of Lin Carter are another strong influence, particularly the Gondwane Epic and, to a lesser extent, the Thongor Cycle; small realms with diverse landscapes, ancient ruins piled on ancient ruins beneath the very feet of rat-on-a-stick vendors, wandering heroes and scoundrels getting caught up in the strange world around them and either getting chewed up and spit our or grasping the tiger by the tail and building their own kingdoms. The science-fantasy elements come mostly via these sources, especially the use of crystals in magic, floating islands, and the pseudo-science of some wizardry... though such is usually masked artistically (i.e., not chrome and plastic, but gold and gems).
Tolkien's Middle-earth: everything else said aside, elves, dwarves, and hobbits (yup, hobbits, not halflings) are all done up high-fantasy Tolkien style. There are a few exceptions here and there, but they are very much exceptions to this rule. However, like the D&D Cartoon Realm, such races are found mixed together in the towns and villages, as well as living separate and distinct.
Judges Guild's Ready Reference Sheets and other such materials in various JG booklets (Village Book, Castle Book, etc.); the three original LBBs; Holmes; the original 1E Dungeon Masters Guide; select pre-2nd Edition Dragon Magazine articles; select tables and articles from Fight On! magazine; select elements of B/X BlackRazor's B/X Companion; and of course, the works of the OSR fans of the Blogosphere, especially Jeff Rients, Al of Beyond the Black Gate, Joe Bloch, Lord Kilgore, and many others, an article here, a mention there. I also plan to add the Castles & Crusades Castle Keeper's Guide to this list once I secure a copy.
Core House Rules
Here are the current House Rules that I'm using with the Core LL Game:
Roll 3d6 six times in order; if the total modifiers of all six abilities do not equal +1, you may reroll all six;
Your starting level is d6: 1 = 1st level, 2-5 = 2nd level, 6 = 3rd level;
Hit Points at 1st level are equal to your Constitution score plus your class Hit Die plus Constitution Modifier; each level thereafter goes up as normal;
Alignments are: Lawful Good, Lawful, Neutral, Chaotic, Chaotic Evil; alignment is as much an alliegance to a Force as it is a moral/ethical issue;
Lawful Good clerics can cast their spells "miraculously," i.e., do not need to memorize their spells, but still must pray every morning for their spell slots. Lawful clerics gain and memorize spells normally. Neutral, Chaotic, and Chaotic Evil clerics must maintain and use a Prayer Book of spells they are granted by their Hierarchy; to add a spell otherwise to this book is Heresy.
Lawful Good and Lawful clerics can only cast spells in a "good" fashion, i.e., the "normal" version of spells (note the "good" version of animate dead is destroy undead). Chaotic and Chaotic Evil clerics can cast either version of a spell, but of course, rarely use the "good" version of a spell except to save themselves or chosen minions. Chaotic and Chaotic Evil clerics can also use edged weapons.
Neutral clerics cannot advance beyond 7th level.
Clerics can belong to one of the organized temples or not; with a temple, they get hierarchical support, without they are on their own.
Fighters get Weapon Specialization in a weapon of choice at 1st level; +1 to hit and damage OR +1 to AC (if weapon appropriate), choose before Initiative is rolled each round; increases to +2 at 3rd, +3 at 6th, and so forth.
Magic-users get bonus spells for their Intelligence as clerics get bonus spells for Wisdom. Each spell they have memorized also has an attendant Cantrip that can be used again and again (though rarely with certainty) as long as the spell is memorized. In Core MU spellcasting remains otherwise Vancian; I'll use a Spell Slot Power Point system in the AEC Campaign. Magic-users can use any weapons they choose, but only do 1d4 damage with 1-handed weapons; they do 1d6 damage with 2-handed weapons. They can wear armor but not cast spells while wearing it; the armor's AC is one worse, and for encumbrance, it counts as one worse (leather as metal, metal as 30' (10')).
MUs have a spell book for every spell. If they do not have their spell books, they can only re-memorize spells they cast the prior day (Vancian style). All MUs start with a townhouse, small tower, hut, cottage, etc. as their base of operations, needful for the large magical paraphernalia needed to research and maintain their spells (c.f., Shimrod's cottage in Vance's Lyonnese trilogy). They also have a manservant/minor animate/permanent unseen servant/demonling/ etc. to serve them in basic butler/maid/chef capacities at their home.
Thieves in my AEC Campaign will get a wide variety of skills they can choose from, but the Core campaign skills are by the book. They can wear any armor, but it affects them like it does magic-users; if they try to use a skill while wearing it, they suffer a 10% penalty per point of AC the armor provides better than Leather, and if they fail, they fail spectacularly.
Elves get bonus spells as per magic-users, but can only choose one Cantrip per spell level rather than getting all cantrips for all spells memorized. Elves get weapon specialization but can only choose from sword, bow, or spear.
Dwarves get weapon specialization with axes AND hammers OR crossbows.
Hobbits can choose weapon specialization with one weapon OR any three thieves skills that advance as he does.
I have a different pricing scheme for armor; 10 gp for AC 8, 20 gp for AC 7, 40 gp for AC 6, and so forth, up to 2500 gp for AC 0 Jousting Armor.
Characters have three "Loot Carrying Capacities:" Hands-Free/Backpack (400 cn), One-Handed/Small Sack (700 cn), Two-Handed/Large Sack (1,000 cn). We keep track of loot carried and what they have in their hands thusly... the first time they had an encounter while everyone was carrying large sacks was rather comical... the floating disk spell is now considered indispensible!
I keep close track of time in the dungeon and thus of of torches/oil lantern quantities, especially in parties with humans.
Obviously, Resource Management is quite important in my games!
I give 100 XP per HD of creatures slain, overcome, or carefully (not accidentally) avoided, plus GP value of treaure... advancement is at a nice clip, but not blazing, nor classically slow.
We use group Initiative, with PCs then acting either in order that seems most sensible based on the situation at hand or, when in doubt, by Dexterity order.
I use Carl Parlagreco's "Good Hits & Bad Misses" critical hit charts from Dragon #39; Critical Hits are on every Nat 20, Fumbles on every Nat 1; PCs always get saves against really bad results, as do Major Villains.
I use a variant of Jeff Rient's Death's Door rule for PCs and Major Villains.
that's about it for now...