The manual for The Witcher 2 lists the various difficulty level, most of which seems to be pretty standard. The last one did catch my eye though:
Dark – for very experienced gamers. This difficulty level allows you to acquire dark items and grants you access to quests which are not available on other difficulties.
Although it is very unlikely that I'll actually be able to play at this difficulty level, I am rather curious about what these quests and items are, and how they affect the story (use spoiler markup as necessary).