There are no "classes" in CoC; everyone's pretty much a normal joe who has the ability to accumulate skills, feats, hit points, and levels on a common basis with all other characters.
Pretty much, what CoC has to offer a D&D game are the following:
1) New monsters
2) New spells (some of these are really useful, and perfect for a Ravenloft game)
3) Books and artifacts (IMHO, the most useful atmospheric contribution to a D&D game, especially a Ravenloft game)
4) D&D stats for the Great Old Ones (not very useful unless you really want to kill your PCs or your PCs are already 30th level or so, or both)
5) Sanity rules
I don't find Sanity very useful either in a regular D&D game or RL game; I'd rather use the Fear, Horror, and Madness rules myself. Sanity is based on HPL's literary universe, in which protagonists who are accustomed to the rational tone of everyday modern life encounter fantastic alien entities, weird forgotten realms, and strange geometries that wreak havoc with their down-to-earth sensibilities. D&D characters already exist in such a fantastic, unreal universe that it's hard to imagine anything costing them Sanity (unless it would trigger a Madness check under RL rules anyway).
