I'll answer for them. It was annoying that no character acted like a normal human being. Instead, they danced around each other and never actually communicated because that would be too easy and make it way to difficult to get to the final act in order to justify the son torturing his mother to death.
It was also annoying because rather than being new and innovative in any way they merely checked off every horror trope known to man. This movie is incredibly overrated.

What was annoying about It?

You'll survive.

Yep wasted $1.50 at RedBox. plus a few pennies in tax