The Flash utilises a tried and true concept, that of time travel and its paradoxes but doesn't really deliver anything of consequence.
This film doesn't deliver for a variety of reasons. To my mind, the foremost is the lead character. The Flash's over cooked dripiness, unfunny, goofy antics and fall flat humour, can't quite "sell" the concept of him as a hero. If the films producers and writers, wanted to know how to develop a more accessible lead character, they should have looked at the series. It does a much better job at balancing the characters innate nerdiness, with his other, more heroic, qualities.
The second point of failure is essentially re-inventing elements of the story from the series and past films. Flash fans who have watched the series, may well find the film adaptation somewhat repetitive and predictable. Equally recycling large swatches of past films, with tweaks here and there, isn't doing anything that "new" either.
Perhaps the only real point of interest is utilising a range of past actors and characters, from the franchise, such as Batman, Superman/woman and Wonder Woman.
In summary, not an awful film but still a little unoriginal. An outcome not helped by a rather annoying, lead character. This is balanced by decent action and the presence of other hero's from the franchise. That said, the fact its necessary to drop so many cameo's, is telling in and of, itself.