Some devices, like the iPhone/iPad, don't recognize Flash at all, which is another nail in the coffin for Flash. There are nice alternatives to Flash that are native to the browser, so you don't have to reply on plugins.
I don't know what ever motivated designers to have an intro page, though. If I go to an address, surely it means I want to view the site, so why make me have to click an Enter link? I'm glad it's a practice that has mostly died out now.