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.
Regarding his experience and roles - Apparently he's played a villian on Glee (I assume this must be the male version of "Mean Girls"?) and someone mentioned he played twins on a cop show, with different personalities. So, that's 3 very distinct roles, so, there's reason to hope he's up for the challenge acting-wise.
As far as his look, yes, he's quite the pretty boy, but, Stephen Amell has some pictures from before Arrow where he looks quite the Pretty Boy. Also, for many, his size could be a plus, I've seen many people grumble about The Flash (Or Spiderman) being portrayed by too bulky an actor, so, his smaller, toned and athletic, but, not bulky mulscular physique should be right in the wheelhouse for folks who thought John Wesley Shipp was ridiculously buffed for the part. As far as his pretty boy face, there's no reason a pretty boy face can't be taken seriously as a Forensic scientist, and NOTE: The article specifically throws in the word "Assistant", so, he's not even necessarily a fully qualified Forensics Scientist, he could be an intern, still studying.