Innistrad's game mechanics amplify the atmosphere perfectly, and you'll find a LOT of famous horror call-outs in some of the cards.
The most important mechanic is Transform, which is exactly what it sounds like. The card doesn't have a back; the "Day" side of the card is where it starts, then when certain conditions are met, it transforms into the much scarier (and usually more powerful) "Night" side.
You can probably figure out which stories these cards represent:
The werewolves, by nature, flipped between the two consistently. It was always a trick to try and make Night last as long as possible.
It also introduced Curses, which were the first dedicated group of enchantments you could throw directly on your opponent -- not your opponent's cards, but your opponent himself -- to ruin his day. Some of them got super nasty.
I don't want to flood this post with too many more images, but each horror "tribe" had its own flavorful mechanic. Vampires got progressively stronger as they hit the opponent and fed off his blood. Zombies came in two varieties: the Black-aligned "Ghouls" which were the shambling type that swarmed everywhere, and the Blue-aligned "Skaabs" which were stitched-together monsters usually offering discounted resource price but eating a dead creature or two in your graveyard. The Spirits (geists of various forms) didn't have as cohesive of a theme but they were usually small, subtle, and ultimately debilitating if they weren't dealt with.
Oh, and there were demons and cults and stuff too!
The plane was revisited and the flavor turned to more Cosmic Horror as the Eldrazi Emrakul was summoned by a vengeful harbinger to bring ruin to Innistrad. I won't go into much more detail, but I'll leave you what happened to two of the most powerful archangels in the plane. :)
This is some really cool stuff! Amazing gameplay and lore synergy here. ---
