Personally:
Walls of text. If people get the urge to press a button that says skip intro, then is not a good intro.
When they fail to make me care or be interested in the characters. For me, characterization is vital.
If it feels too generic and if they don't give a twist to the apparent cliché. If i feel like i already saw this story many times, then i may leave, except if somehow they made me emotionally invested in the characters.
If the pacing is too slow. I have patience for a build-up for something happening. But if the build-up leads to nothing or if it feels like empty filler, then i lose interest.
Redundance. If they keep repeating the same over and over, they lost me.
Those are the points i remember for now.