It's the fantastic blend of monster FX and dark humour that makes it so much fun. A lot of that is thanks to director Sam Raimi, but also the star of the show, Bruce Campbell, as the excellent Ash Williams.
This film is essentially watching Ash be tormented in a cabin for an hour and a half, so he's got the whole movie to carry. Luckily, Bruce pulls it off, creating a brash but likeable everyman that you can get behind.
Now, good 'ole Bruce isn't the only high note here. There are some excellent special effects to behold. While they can be cheesy or dated at times, they always have a particular charm that perfectly works with this movie's tone. Occasionally, they're even effective in the opposite direction, successfully creeping you out.
Well there you have it, I've told you why a classic is a classic. If you've somehow never seen it, bring friends and/or alcohol and have a blast. This one is a groovy Good!