There's something about seventies horror that reminds me of live theatre, actually. The sets and costumes are often cheap, and when it comes to period pieces, more 'inspired by' than accurate; the makeup is big and visible; even when the effects are really good, the blood is usually unnaturally red. The acting tends toward the broad and stagey.
And yet, it's also clear that realism is not the goal. Rather, the movie works to draw you in to a unified fiction, to get you to share in its nightmare. The best seventies horror I've seen has a dreamlike, Vaseline-lensed quality, a sense that it doesn't matter whether or not everything that happens in the movie is likely or even possible in real life. We've stepped outside of real life into a self-contained bubble with its own logic and its own sense, a dark fairy tale where the corpses of young girls might transmute into hares or eternally hungry floating heads, or the night of All Hallows might summon a stalking, unkillable masked evil from the past, or a ballet studio might be entirely controlled by witches. Even the lowest-budget, most exploitative Hammer flicks don't escape the touch of that dreaminess, that velvety, enfolding unreality. The movie suggests a world, and we, if we are wise, gladly succumb to the power of that suggestion.
Eighties horror, on the other hand, is all about the goo.