In a single word: disastrous. Bryan would have given Jimmy Carter, Franklin Pierce, and James Buchanan a run for their money, individually and collectively, as the most naïve and inept occupant of the Oval Office. (As evidence, I offer his brief, unlamented tenure as Wilson's SecState, during which he spent a significant amount of time sending out evangelical tracts rather than learning statecraft and diplomacy.)
I suspect his bimetallism would have crippled, or at least acted as a significant brake upon, the boom times that were set to take place in the early 20th century. Moreover-and worse-the rise of the US as a true world power would have been delayed significantly by Bryan's proto-isolationism. Further, the progressive reforms enacted by TR (e.g., the creation of the FDA) would likely have been delayed since they would have been a tough sell to what likely would still have been a GOP congress since those reforms didn't come from one of their own.
A good thing Bryan never really got close to the White House.