Depends on the movie. It's a steady plot, good believable characters, development of characters, good editing, and above all fitting dialogue and plot resolution. As a result what makes a movie good is extremely subjective and problematic to really pin down. When I see a movie for the first time I usually know within twenty minutes or not whether I care about the rest of the movie. I have seen good movies that got much better after that point, but it's a rare thing.
A good plot well acted as well as directed.
The blood that squirts out someone's eyes<br />
The screams of terror<br />
Popcorn and soda