To me its not so much his looks as what he's done so far. I don't know if being on Glee translates well to being The Flash
He doesn't have to look that much like the comics version, but he should look like someone you'd accept as a legitimate superhero, this guy looks like a pretty boy from various stupid tV shows (which pretty much is the kind of actor he seems to be). Barry is usually a more serious kind of superhero. This guy looks like he'll be defeated by Mirror master because he'll be too busy adjusting his hair in MM's mirrors
That said, he could be a surprise. Maybe they'll make him look more like an actual forensic scientist and he'll turn out to be a good actor. I hope thats the case, and I'm more than ready to give Arrow the benefit of the doubt. I just won't be super surprised if he fails at being a good Flash.