Although it was yesterday, I don't think anyone will hold it to me that I'm contributing 24 hours later. The truth is that I have much more playing experience with Labyrinth Lord than Swords & Wizardry, although the two games are similar enough that I poached plenty of material from the latter. The things I like most about Swords & Wizardry may be far different than other bloggers' thoughts, but I feel that they work.
Race and Class: In most OSR games, nonhuman races are locked into a type of role; there are no dwarven thieves or spellcasters, and elves are fighter-mages. Although this has been preserved to an extent in the Core Rules and White Box, S&W Complete and Core actually separate race from class. Instead of locking dwarves, elves, and halflings to their own full class, it presents three or four qualities for their races. A halfling cleric or dwarven thief isn't really game-breaking, and it provides an interesting springboard for making your own races.
The Single Saving Throw: Labyrinth Lord and Lamentations of the Flame Princess have 5 saving throws. 3rd Edition and Pathfinder 3; 5th Edition has by far the most, with 6 saving throws. Swords & Wizardry boils it down to 1; and to simulate different key resistances, certain classes get a bonus on said Saving Throw against certain phenomena, like traps or spells.
Again, the single Saving Throw is a minimalist yet elegant design element, while still allowing for some variety with classes getting situational bonuses.
Comprehensive Projects: Labyrinth Lord may be the king of 3rd party OSR projects, but the ones that exist for Swords & Wizardry tend to be large and broad in use. A large part of this is thanks to Frog God Games, whose Lost Lands campaign setting gave us sandbox adventures such as the Sword of Air and megadungeons such as Rappan Athuk and Slumbering Tsar. There are also 2 large bestiaries full of several campaigns' worth of monsters, Swords & Wizardry Monstrosities and Tome of Horrors Complete. The latter updates a lot of classic D&D staples from earlier official materials such as the Fiend Folio, so it's got hundreds of converted material ready-made for you.
Then there is also White Star, a space opera retroclone which has a lot of nifty ideas and support from many publishers. So the third party support gap might be closing up after all, if more due to sci-fi material.
Outsider-Friendly Options: By 'outsider,' I'm referring more towards mechanics which make it easier for players of other Edition and retroclones to jump right in. Namely ascending armor class and Challenge Levels for monsters. One of my major critiques of OSR retroclones was the use of a to-hit matrix for combat, which was a lot more cumbersome than the ascending AC of newer games. Lo and behold, Swords & Wizardry had a good idea with this rules variant, allowing you to just roll a d20, add relevant modifiers, and compare the result to the enemy AC. I found that this was a lot easier to grasp for gamers from Pathfinder, and to continually use for campaigns.
As for Challenge Levels, the Challenge Rating system of 3rd and 4th (and likely 5th) Edition gets a lot of flak, but used ideally it's a good abstraction for a ballpark estimate of monstrous power. Swords & Wizardry's system is actually keyed to the 'dungeon level' format, where the deeper levels tended to have stronger monsters, and was built from there. So with Challenge Levels for monsters from the core and new sourcebooks, you can get an inkling of when to deploy that monster in your campaign.
It's Got a Nifty Compatibility Logo: Specifically, the one I'm thinking about from Gamers & Grognards Studios. It's free and open source for self-publishers to use, but its design is quite eye-catching: