Well I'm a bit too tired to think of any original answers, lol (although I'll be back tommorow) but I'd agree that judging by those initial IMDB scores; Big Fish, Signs and Road to Perdition are all very overated.
Signs was a nice, decent little sci fi film with drama at the heart, but that kind of rating is ridiculous. When you see some of the tripe in IMDB's top 250, and some of the real gems that aren't up there...well, lol.
Big Fish isn't that bad, but to me, it represents that type of film that wavers between the classic it might have been and the absolute catasrophe it could have been. It's not a bad film, but it strikes me as being the kind of film a lot of people think they "should" like, and as such, rate it so highly, probably without ever seeing it.
Road to Perdition is a tricky one. It's a nice film, slick and well acted, but is it memorable...not in my opinion, and I think that's the most important thing. I remember bits and pieces from various scenes, but there's nothing revolutionary about that film, nothing incredible, or fresh or groundbreaking. Not saying a great film neeRAB to have all those qualities, but once again, I think it falls into that category of films people think they should like, based on the subject matter.