Oh, where to begin...
Let's start with casting. Ezra Miller looks nothing like Barry Allen, and he acts nothing like the Barry Allen in the comics/animated movies. He plays Barry as extremely socially awkward and can't deliver a joke to save his life. I don't know how he researched the role, but I don't recognize the character in his performance.
Okay, the plot. It's structure is Flashpoint, with a hint of Crisis on Infinite Earth's, and a spin on the Once and Future Flash. I would have enjoyed it more if it was true to Flashpoint. Wonder woman vs aquaman, Thomas Wayne, Reverse Flash, etc. Instead, it was a bit all over the place, and it seemed like the plot was structured around the cameos, of which there are many.
The CGI. Yeah, the director says it's intentional, but it looks like garbage. The suits are noticeably bad. The chronobowl scenes are god awful.
The action.. The coolest action scenes belong to Keaton's Batman and Supergirl...in a movie titled THE FLASH. Even the final battle had mediocre action from the Flash.
Other characters from Flash's backstory were barely used, and when they were they didn't resemble anything like the comics. Patty Spivot comes to mind. Iris West had about 5 minutes of screen time...
As a Flash fan, this was such a disappointment.