With decent writing I can see Reynolds making an awesome Flash. In other stuff I've seen him in he does that quietly sarcastic thing perfectly, so I could see it working.
To be honest... I don't why this is, but I kind of want a younger actor to be the Flash. He's never really portrayed as a young guy (at least not in the limited Flash stuff I've read) but it just seems right to me that someone with super-speed be a young and impetuous kind of guy.
That would be cool, yeah, I like that idea. Kid Flash has a lot of personality in the Young Justice cartoon and other animated stuff I've seen, but he's a complete tool in the newer comics, especially Flash: Rebirth. Anyway, a young, loose canon style Speedster would be really cool to see. I like it when heroes are explored in different emotional ways, like in Hancock