So, I've had about 24 hours to peruse Carcosa amidst writing essays for finals and thinking up puppet-master tables. Here's some thoughts:
There's an encounter with barbarians in the module at the end of the book, which isn't terribly significant in the grand scheme of Carcosa, but here's two things I gleaned from it: the first is a concise way to get a feel for Carcosa. 30 purple barbarians riding bright red allosaurs and led by a red man wielding a pulse rifle, out hunting for those that worship or in any way interact with the Old Ones. If you need a one-sentence image to communicate Carcosa that doesn't dwell on the sex, torture and human sacrifice, that'll do it. I'll leave whether you want to do that up to you; since I'll never run Carcosa as-is, but will be cherry-picking from it, I don't see that I'll ever really need to give someone a mental image of Carcosa, but there it is. I suppose if you wanted to strip the vile aspects of Carcosan sorcery and then run Carcosa as-is, that would work.
The other thing I noticed is the mechanical descriptions of the barbarians: they have an attack bonus of a Fighter two levels higher than they are and they never have to check morale. A criticism I think is valid is that Barbarian classes usually involve mechanics like raging/berserking that take control of barbarian characters out of their players' hands. The mechanics in Carcosa got me thinking about taking the opposite tack: how about when every other character needs to make a save or have mechanics take over control of the character Barbarians don't need to worry? The obvious situation would be save vs. fear situations and when some big bad causes awe in its enemies that make them roll all their dice with negative modifiers. It's probably pushing it a bit far to make a Barbarian immune to things like Charm or Sleep, though.
One thing I don't get: Carcosan sorcery is really horrible, but what do the sorcerers do with the Old Ones once they've conjured and subjugated them to their will? I mean, what are the Old Ones good for? I suppose a sorcerer could turn them on a settlement, or maybe an alien base, to destroy it, and sorcerers can get information from them, but nothing in Carcosa that I've read so far indicates that the Old Ones know anything in particular besides details of sorcerous rituals. I suppose that a referee can have them know anything that a sorcerer would be interested in knowing, and sorcerers can find lots of uses for Old Ones if the ref/player wants them to (the "why have us do any more of our imagining for you?" argument); it's just that I'm having trouble believing that sorcerers would mess with the Old Ones without very compelling reasons to, seeing as how they're so dangerous and all, and how kidnapping, raping, torturing and killing other people's kids tends to lead to them wanting to hang your "carcass from a tree," to quote the desires of the barbarians mentioned above. Maybe I haven't read enough Lovecraft (though I haven't ever heard of any Lovecraft stories that involve subjugating Old Ones to a character's will- if you know of one, please let me know), but I can't think of many compelling reasons to do that in Carcosa, at least not off the top of my head; warfare is about the only thing that comes to mind, but Carcosan sorcerers are adventurers, not ranking members of villages, so that isn't totally satisfying. Ah, well, if the PCs stop and kill a sorcerer before he can complete his ritual or be interrogated, he almost doesn't even need a reason, now does he?