I am not sold on the DCU yet, but the first trailer is like the first impression, it lasts damn long.
Which is no doubt why they're taking the time to make sure they're satisfied with the trailer. It's so puzzling to me when people react to news about creators doing more than one draft of a thing as if it were some shocking warning sign of looming disaster, rather than a completely normal and healthy part of the creative process.
(Although I was always taught that you shouldn't trust first impressions.)