Hi everyone. This looks like a great thread, wish I had noticed it sooner !
I bought the Manual of the Planes 4e, and it was a good read. I'm suprised that nobody has mentioned Shadar-Kai, The Raven Queen, or Fell Taint yet, as those were key parts to the presentation of the Shadowfell. I'd like to share my worldview of the plane, and I am unsure if Ravenloft could fit.
Shadar-Kai were a race of humanoid PC characters who lived in the Shadowfell. They were kinda like humans who lived in the plane for so long that they gained resistance to the necrotic forces, and they also gained a shadowstep ability. They wore gothic attire, and wielded spiked chains, daggers, and other such metallic weaponry. They are commonly understood to hunt undead, either because of the undeath threat on the plane, or because they would be conscripted by followers of The Raven Queen. They have adapted pretty well to preserving their souls as well as laying to rest other haunted souls.
The Raven Queen was the mysterious god meant to watch over the Shadowfell and all the souls that passed through the plane. Whenever a person dies, their soul passes briefly through the Shadowfell before going on to their final destination. Kinda like a limbo or sorting area. Though it worked well, things could go wrong during your journey to the heavens ! The Raven Queen made it her duty to preserve this natural order, and sent her followers to hunt all sorts of ghouls, witches, and demons that would seek to harvest those souls for foul acts. There was even an adventure path where Orcus sought the true name of The Raven Queen as a weapon to use against her.
Fell Taint was a general environmental hazard throughout the Shadowfell. All the death, departed spirits, and necrotic energy could do nasty things to you. If you stand in the wrong area, you could suffer necrotic damage, receive extra damage from skeletons, or even spontaneously turn into a wraith ! Of course, the Shadar-Kai and other live residents would avoid these environments, but one must always be wary of the threat it imposes. With all the Black Taint, stray souls, and the demons who are hunting them, well some places in the Shadowfell are just plain off-limits!
I suppose that the easiest way to reach Ravenloft would be through the Shadowfell, but I'm not sure if Ravenloft exists on the plane, or is somehow separate but connected to the Shadowfell.