Everyone is entitled to their opinion. There are more reasons than just his hair and eyes. I don't see the same thing when I watch him as you do, he's not that impressive to me. Nothing about him stands out to me. Now, I'll admit, I've noticed in the last few years that my preferences in terms of acting don't line up with the general audience as much as they did 5-10-15 years ago, so maybe he is primed for stardom but I don't see it.
I also don't like that The Flash is going to be 15+ years younger than Batman. That's too big of a gap. You could brush that off as just my preference but regardless, I don't like it.
Just because those of us that don't like this casting choice are the minority, doesn't mean we should keep quiet or that we're automatically wrong because most people disagree or because Miller is a respected actor amongst "experts" or the general audience.