I could list a thousand little things.
It was Bill and Ted's Excellent Adventure as the Flash. Stupid, over the top, comedy relief. Overwhelmingly bad CGI. Terrible plot. Terrible execution of the story.
What was good? Cameos. Oh look, Michael Keaton and the Batmobile.
Spoiler:
Any questions on why I think this... see the opening scene. Therapy dog. Microwave. Batman causes crazy damage to the city chasing bad guys for 20 minutes while the speedster is being comedic relief. What would it be, 3 seconds for the Flash to catch the car and pull out the driver?