I watch horror, so its usually a combination of these, in no particular order:

• Believable plot.
• Believable character actions.
• Lack of common tropes/clichés.
• Quality of filming (angles, shots etc. Not resolution/film quality).
• Atmosphere.
• Tension/unease, and the handling of such.
• Quality and predictability of scares (if any).
• Good pacing.
• Bonus points for layers/deeper meanings.
• Bonus points for good visual effects, e.g. CGI.
• Double bonus points for good practical effects.
• Triple bonus points if I’m scared.