What follows isn't a review but an observation based on reading the original un-purged version of Geoffery McKinney's Carcosa.
Carcosa taught me a lesson in campaign design, one which I fear many of us playing fantasy RPGs have forgotten. Magic should be magical. Granted the magic in Carcosa is certainly dark and terrible and casts deep shadows on the entire campaign if one ponders them too deeply but it is certain that rituals of the Carcosan sorceror are Carcosan Magic.
The magic within that setting defines it and is built into it. It feels like magic that belongs there. When a sorcerer is involved in dark rituals, well the sorcerer most certainly is involved in dark rituals. Rituals that are written into the campaign. There are places that must be visited for their bits of lore, beings to be dealt with and rare goods to be gathered that are campaign defining.
In Carcosa the magic isn't a bolted on extra it's part of every character and is woven into the land. Too many games and campaigns treat magic as just another option another set of gear, skills or FX, just some gloss to make 6d6 damage a bit more stylish. It could be more, it could be a part of the campaign woven into the landscape and into the beings of the campaign.
Every setting need not be dark as Carcosa and magic doesn't have to be terrible but I can't help but imagine as immerssive a treatment of magic could work for a great many campaigns. Magic should be noticeable and mean something. Players of Wizards shouldn't skate by treating the acquisition of magic spells as if they were hunting down mere recipes. Telling someone you are a wizard shouldn't get as little notice as telling someone you are a carpenter. The practice of magic shoudl mean somehting within the campaign.
So when next I am working up a fantasy campaign, magic shall actually be a part of the game beyond the concerns of one or two players in the group and a set of powers to bolt onto villain #105. I want Magic to be Magical and makign magic a definign element of the campaign looks like a lesson learned well from readimg Carcosa.