At this point, it's hard to surprise anyone with the idea of a hero moving at superhuman speeds. Man of Steel showed the feat performed by heroes and villains alike, Smallville featured Clark Kent as 'the Blur' for multiple seasons, and most Marvel superhero movies have also included it in one way or another.
But The Flash's increased speed, perception, and even thought process mean that a majority of the film's action will take place at super-speed. Settling on a mere blur swiping across a scene or slow-mo backgrounds and a normal-speed Flash won't cut it: the filmmakers will need to do something genuinely new (and expensive) to keep fans engaged.
If audience members can experience super-speed in a new way, the bar will be raised for every hero that can do the same; not to mention the mind-bending visuals of Barry's speed warping the fabric of space-time itself.