Let's get this out of the way: This television version of Charles Dickens's classic story is not the best acted nor the best produced, and it's special effects are (quite frankly) laughable even by the standards of the time. It's gifted actors seem like they're rushed to deliver all of their lines in the course of a commercial television production (because they are). None of that is what makes this such an outstanding version of the teleplay.
The secret sauce here is that this is filmed in 1970s BBC studios, and that means you get the wonderful effect of the PAL video technology at the time, with its overblown highlights, deep blacks, and high refresh rate. These qualities inherent in the technology of the time make this (in my mind) the spookiest version of this story ever put to screen. The ghosts are just more ghostly, and matte-background London looks much more gritty than any other version. The atmosphere is palpable and feels oppressive, and that is what you want in what is arguably the most famous ghost story ever told.