I'm an Edwards supporter now, and why he has been held back is that not everyone is considering him yet. If he can get the media coverage to be a choice between Kerry or him, I think he has a shot at it again. If the media still talks about Bush v. Kerry, Edwards will suffer.
On the contrary, Edwards' main plus is that because no one is attacking him, he is free to run a positive-message, no-attack-ad campaign. The reason I voted for him is precisely because of that. If the Kerry campaign starts to perceive Edwards as a threat, they will start slinging the mud his way. Then his choice will be to sling back, and lose his upside, or refuse to repay evil with evil, in which case he will be buried.
Right now his best chance is to keep the focus on Kerry v. Bush, and quietly pick up support with is positive message as a candidate for the Little Guy.