Underutilized setting sources in canon

[TheArcaneWeirdo]

I have to admit I'm unfamiliar with most of the settings discussed here.
Ravenloft itself is the setting I'm most fluent in.
Most of my RPG friends - and thus most of the adventures I've actually played - prefer homebrew settings and mechanics.
I forced myself through the Dragonlance novels to understand references etc but wasn't too impressed (It was full of cool concepts for characters, locations etc, but the writing itself was often bland. I noticed it's a common problem in RPG based novels: RPG writers, beings DMS first and authors second, know how to worldbuild and how to write action, but their plots are written from a 'what would be fun to play' perspective - limiting diversity, depth and lore - and the prose is just plain and generic. Actual Play Podcasts compensate for this weakness by recording the party in real-time rather than after-the-fact, playing fully for the strength RPG has over other forms of media: The spontanity, the chaos, the party's real-life dynamic...)
I've done some research about Forgotten Realms (a lot of random wiki pages & some of Ed Greenwood's YouTube videos), and watched once a huge video about Eberron made in celebration of 'Forge of the Artificer' when it came out, and read a little bit about Greyhawk because of Ravenloft (to understand where Azalin came from), but that's about it.
Can I get some explanation about the other settings?

[The Dark]

I have to admit I'm unfamiliar with most of the settings discussed here.
Ravenloft itself is the setting I'm most fluent in.
Most of my RPG friends - and thus most of the adventures I've actually played - prefer homebrew settings and mechanics.
I forced myself through the Dragonlance novels to understand references etc but wasn't too impressed (It was full of cool concepts for characters, locations etc, but the writing itself was often bland. I noticed it's a common problem in RPG based novels: RPG writers, beings DMS first and authors second, know how to worldbuild and how to write action, but their plots are written from a 'what would be fun to play' perspective - limiting diversity, depth and lore - and the prose is just plain and generic. Actual Play Podcasts compensate for this weakness by recording the party in real-time rather than after-the-fact, playing fully for the strength RPG has over other forms of media: The spontanity, the chaos, the party's real-life dynamic...)
I've done some research about Forgotten Realms (a lot of random wiki pages & some of Ed Greenwood's YouTube videos), and watched once a huge video about Eberron made in celebration of 'Forge of the Artificer' when it came out, and read a little bit about Greyhawk because of Ravenloft (to understand where Azalin came from), but that's about it.
Can I get some explanation about the other settings?

Going through some of the ones I'm more familiar with:

Mystara was the setting for Basic Dungeons & Dragons. It's very much a "kitchen sink" setting, with the not-Roman Empire, the not-Mongol horse nomads, etc. A number of the re-done "classic" adventures were originally for Basic, like the Isle of Dread and Castle Amber. It's a very high-magic setting and had rules for characters to advance all the way to being Immortals (the setting-equivalent of gods). It also has a Hollow World that's used by a group of Immortals to preserve dying cultures that they're interested in, so it has Neanderthals and Ancient Greeks and Aztecs and Egyptians all mixed together.

Birthright was one of the later settings. It was trying to bring the game back to the wargaming roots by setting up PCs to be the rulers of countries or the heads of temples or guilds. It also had a backstory where the original gods had destroyed each other in battle, and their divine essences both created new gods and empowered people with divine connections to the land. Some of the most powerful bloodlines will turn people into inhuman creatures, which include some of the major villains of the setting but also had some heroes added in later supplements. It also has a corrupted fae world that was the original home of its halflings and can be accessed through portals (halflings can travel back and forth more easily than other characters). It has five human cultures that are basically Chivalric France, Scotland, Low Country Merchant-Princes, Medieval Caliphate, and Russia. Its dwarves are fighting a mysterious war against underground foes, while its elves were the original inhabitants of the continent and mostly hate humans for encroaching on their land.

Jakandor was specifically designed to be a bottle setting that stood alone but could be dropped into existing campaigns, since it takes place on one island. The two cultures that are fighting each other are one oriented around different types of fighter, the other around different types of wizard. There were plans for a second class-based setting that would have been urban and based on clerics vs. thieves, but it never got published due to TSR's disintegration. Jakandor heavily relied on 2e's kit system, providing 20 kits for each culture - 1 for their "opposed" class, 4 or 5 for each of their "neutral" classes (clerics and thieves), and either 9 or 11 of their "preferred" class (9 wizard kits and 11 warrior kits). The Charonti had kits for 9 wizards, 5 thieves, 5 clerics, and 1 fighter while the Knorr had 11 fighters, 4 thieves, 4 clerics, and 1 wizard.

Dark Sun is post-apocalyptic fantasy that got very heavily retconned during its run, so (somewhat like Ravenloft) people need to be clear which boxed set and which sourcebooks they're using when discussing the setting. Dark Sun's magic tends to kill plant life, and the various rulers of its city-states are wizard-psionicists who are becoming dragons. It's heavy on psionics and was originally intended to promote the BattleSystem miniatures rules, which died off after the first few supplements. It uses the standard AD&D races but with twists (elves are tall desert nomads who run fast and don't think their moral codes apply to non-elves, halflings are barbarians who are rumored to eat anything that's not another halfling), plus a few new races like muls (half-dwarves) and thri-kreen. The first set of five novels (the Prism Pentad) heavily changed the setting, and a Revised and Expanded setting was released after they were done with all of its updates. It got a 4e version that reset the timeline to after the first novel but before the rest of them.

[IanFordam]

Al-Qadim is the Arabian Nights-themed setting, and I find it richly detailed and flavorful. While the traditional humanoid races are widely present, the stereotypical divisions are not. As long as you're Enlightened, you're accepted.

Officially Al-Qadim is set in Zakhara, a region within the Forgotten Realms. However, the ties to the greater Realms are minimal, which I see as an advantage. The setting is strong enough to stand on its own.

[The Dark]

Yeah, for 2e, Al-Qadim is very open to (at the time) non-traditional player species. City of Delights had rules for goblin, kobold, orc, hobgoblin, ogre, and half-orc and half-ogre PCs. Some of the NPCs include a goblin barber, a set of ogre brothers that work as porters carrying heavy loads, and an ogre merchant-rogue.

Some of the potential struggles in the setting include city-dwellers against nomads, the zealots of the Pantheist League against the less traditional Pearl Cities (they're across a bay from each other), the Enlightened against cults of Unenlightened deities, Corsairs against Mamluks, and the nefarious plots of the yak-men.

One of my favorite minor connections to other settings is that one of the captains on the corsair council is a renegade spelljammer from Kara-Tur.