What are the differences that a player or GM would notice when going from one system to the other?
I sat down with both PDFs this morning and compared them system-by-system, drawing on my experience with B/X D&D and Adventurer Conqueror King System to zero-in on the parts of early D&D that frequently vary and make the most difference. These notes apply only to core Swords & Wizardry (not White Box), and core Labyrinth Lord (not including either Companion). The PDFs are available for free online, so grab a copy of Swords & Wizardry Core Rules and Labyrinth Lord Core Rules (complementary no-art version) and follow along!
Labyrinth Lord cleaves relatively closely to the rules of Basic Dungeons & Dragons and Expert Dungeons & Dragons. The rules are reorganised for clarity and to not be divided between two volumes, and revised in a few places to resolve ambiguities. LL was written with the goal of creating an in-print reference system for published material to declare compatibility with "Labyrinth Lord" and have everyone know that they really mean it's compatible with B/X D&D.
Swords & Wizardry is faithful in style and feel to original Dungeons & Dragons as published in the original three brown books and the following supplements. However, in many structural ways it takes inspiration from Dungeons & Dragons 3rd Edition. To players it will feel very much like the original game; to referees it will feel like the original game rewritten with some of the DM-side conveniences of 3e in mind.
SW has split race/class choices and non-humans may (but not must) multiclass. LL has race-as-class.
LL uses a consolidated attribute bonus scheme, where (for example) your to-hit bonus from strength will be the same as your damage bonus from strength. SW uses a scheme more true to the original, where an attribute gives you different bonuses to different things, according to the attribute tables. Bonuses in LL range from -3 to +3 and are standardised across the attributes. SW gives different bonuses depending on the attribute. For example, DEX bonus to AC in SW ranges from -1 to +1, instead of hewing to a standardised range -3 to +3 range as in LL.
In SW, strength bonuses to hit and damage are for fighters only (by default). LL gives them to everyone. SW strength bonuses are lower.
1st level clerics get spells in LL. In SW they do not gain spells until 2nd level. (SW is more faithful to pre-AD&D editions in this.)
In LL, the XP totals for each level all end in 1. So a fighter in LL needs 2501 XP for 2nd level. In SW, the same fighter would need 2500 XP.
SW allows humans to dual-class (abandon their first class to begin in a new class).
LL has minimum attributes for classes, while SW dispenses with that concept. Both gives XP bonuses XP for high prime attributes, but differently: LL gives a max +10% and a minum -10%; SW gives only positive XP adjustments, and also gives PCs with high wisdom a bonus regardless of class, making the max bonus +15%.
LL magic-users and elves get two 1st-level spells and one 2nd-level spell for starting spells. SW magic-users can potentially start with as many 1st-level spells as the PC can know according to INT, but which and how many are known is determined by rolling their “Learn Spell” for each spell, so they will likely start with fewer than the maximum.
LL thief skills are rolled on a d20. SW saving throws are rolled on percentile, except for Hear Sounds which is on a d6.
LL has a Law-Neutral-Chaos alignment system. SW uses this by default, but notes that the DM may just as easily use a more complex alignment scheme.
LL has an inheritance rule, allowing PCs to have a designated “heir” that they can leave money for, which will be gained by the player’s next character in case of the death of the first.
Starting gold in LL is 3d8×10. In SW it’s 3d6×10.
LL uses descending Armor Class. SW uses descending AC by default but allows for ascending AC.
Encumbrance is in pounds in both. SW allows for about twice as much weight carried than LL before movement rates are modified downward. SW also modifies encumbrance limits by strength. LL is close to B/X in this regard, while SW is closer to 3e.
Fighting and Dying
The order of combat in each differ significantly. SW also gives an optional alternate order of combat. Both LL and SW use group initiative. LL (and SW alternate) divide spells, missile attacks, melee, and movement into separate phases. SW standard resolves all spells, attacks, and movement for a side all at once. LL resolves each phase in order, but all phases for a side before the other side goes. SW alternate resolves actions by phase, with each side acting in each of the phases according to DEX score.
LL uses the melee concept of being “engaged”, requiring the use of combat movement rules in order to attempt to disengage. This makes it possible to block opponents and to prevent enemy movement, much like 4e’s fighter powers are intended to operate (but vastly faster to manage). SW doesn’t use melee engagement rules. Instead, character block a 5’ section. Movement in melee isn’t given rules, but SW suggests free attacks against characters/monsters moving within melee or retreating.
In LL combat, natural 1 is an automatic miss and natural 20 is an automatic hit. In SW, neither is automatic. Neither have rules for criticals.
Missiles attacks into melee: In SW the target of a successful attack is random (including friendlies). LL makes no mention of how to adjudicate firing into melee.
Damage in LL is either 1d6 or variable by weapon. Neither is given “official” or “optional” status, so DMs are left to choose which way to handle weapon damage. SW uses variable damage.
Melee range in LL is 5’, while in SW it’s 10’.
SW includes a parry or “fighting on the defensive” rule for fighters, optionally for everyone.
Turning undead in LL is easier for a given HD of creature but SW allows for the possibility of turning more powerful undead. E.g., a LL 1st-level cleric turns a 3HD undead on 11+ but can’t turn 4HD undead; a SW 1st-level cleric needs 16+ to turn a 3HD undead but has a chance (19+ and 20+) to turn 4 and 5HD undead.
Creatures die a 0hp in LL. In SW, 0hp is unconscious and bleeding out at 1hp per round, and a creature dies at negative hp equal to level.
Healing in LL is 1d3hp per day of rest. In SW it's 1hp per day of rest, but four weeks of rest returns a character to full hp regardless of how many they’re down.
LL has morale rules that involve rolling. SW leaves morale up to the DM’s judgement.
SW includes mass combat rules.
SW spell lists are slightly expanded compared to LL, but only by about two spells per spell level for clerics. For magic-users, the selection of spells in SW is about 50% to doubled over LL, depending on the spell level. Many of the additions are from or inspired by 3e.
As an example of how the spells effects are approached differently, Charm Person in SW operates as in the First Edition: targets become thralls until dispelled and there is not much text devoted to what that means, and the spell is generally considered to be quite powerful. In LL there is a goodly amount of text for exactly what Charm Person does, and the target gets regular saving throws to regain control of themselves.
LL monsters are pretty much straight from Basic and Expert D&D. SW monsters are comparable but use a stat block that is unique to SW. Notably, treasure type is omitted, and the variety of creatures doesn’t match B/X. This means that 3rd-party materials designed for B/X compatibility or which rely on the referee having easy reference to specific creature stats (such as “the mutant orcs fight as neanderthals” or just assuming you have stats for a giant weasel handy) will be harder to use with SW. However, the selection in SW will be more familiar to 3e DMs, and it will be easier to use 3e materials without writing new monster stats with SW than with LL.
SW has detailed rules for statting up new monsters, which will feel familiar to 3e DMs.
XP for defeating a creature is different between LL and SW, though not reliably more or less for one than the other.
LL uses a traditional Treasure Type to randomly determine the value and contents of creature and unguarded treasure hoards. SW bases the value of a creature’s treasure hoard directly on its XP value, and then gives a system for randomly varying the contents.
LL magic items appear to be traditional B/X selection and the random-generation tables are pretty traditional as well. SW magic items appear to be inspired by 0e magic items, but the variety is blended with 3e-style modifiers such as “+1, +2 versus specific foe”. Again, this would make LL much easier to mesh with materials and dungeons designed for B/X than SW, and vice versa for materials and dungeons for 3e.
LL calls seeing in the dark “infravision”, while SW uses the D&D 3e concept of “darkvision”.
LL has the traditional spread of saving throws (Breath, Poison/Death, Petrify/Paralyze, Wands, Spells). SW has a single saving throw number for each class. SW does have the traditional saves as an option, but notes that there are parts of the system that the DM would have to alter slightly if using that option.
LL random wilderness encounter checks are made 3 or 4 times per day. SW random wilderness encounters are once per day. This significantly influences how often and even whether you will see players going “nova” or tending toward a 15-minute adventuring day, but is trivial to houserule to taste.
The encounter tables are different in more ways than can be easily enumerated. LL uses a traditional “Dungeon Level” ranking for random monster rolls, while SW uses a “Challenge Level” system that looks significantly different in results and somewhat derived from 3e’s Callenge Rating system. LL’s wilderness encounter tables are “flat”, with a pile of possible creatures for each terrain. SW uses tables to determine creature type instead, which refers the referee to another table to determine the exact creature of that type (eg, a hill encounter with “animals” could be with “apes or bears” which is then left up to the referee.)
The sample dungeon in SW is bigger. The sample overland area in LL is bigger, but the map isn’t as nice as the sample overland in SW.