There's an old piece of industry wisdom that says, when casting your leads, the audience has to either want to be them or sleep with them. (Actually, they use a coarser term, but my editors do so hate it when I lead off with an f-bomb.)
This doesn't solve the inexplicable mystery known as Shia LaBeouf's career — my theory is he's a low-wattage avatar that doesn't distract from his films' real stars, the digital effects — but it's true more often than not.
Just take a look at two films on release this week, "Iron Man 2," with Robert Downey Jr. back as the armor-clad superhero, and "The Last Song," with tween idol Miley Cyrus in her first "grown-up" role.