Even when one knows about the characters, I still would have a lot to be surprised, what moves do they have? how are their personalities displayed on the screen? Do they have appropiate voices for the characters? What cool costumes do they have? Does the trailer do justice to the character? What winks are there to the source material?
I suppose it mostly depends on what you consider valuable though.
Oh yeah, forgot about that. The leaks rarely show information on those kinds of things. Still, I think the surprise can be ruined. Maybe not fully, maybe not so big, but it is still there. You know what I mean, right? I mean, lots of people as I have seen are still going to be excited either way, but I do think that knowing beforehand may have some impact on their excitement when they get revealed. But like I said, the ruining of the excitement doesn't have to be big.