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
Right, because actors' only ever do one type of TV shows. Nobody's ever gone from a goofy comedy to a serious drama, or from a teen series to an action series.
And every one who has done that was awesome, right?

I'm sorry, but Glee is not a show that you should expect good actors to come from. Its not impossible, but his work to date combined with his age and how he looks he's definately not someone I have a lot of confidence in when it comes to playing Barry.
I'm not saying its not possible for him to be good, I'm saying every image I've seen of him makes him look like a stupid pretty boy and I can't imagine him being a guy who works with forensics. I want to be proven wrong, and I'll watch his arrow episodes with an open mind, but I won't be too surprised if his casting proves to suck and it ruins the chances of a flash TV show.