I think Keenum has a mental toughness that Plummer never had. Nothing was handed to him. He was undrafted. He took his lumps as a backup on some truly crappy teams. He's either going to be like Matt Cassel and cash in on his one season in the sun or he's going to be like Tony Romo that flourishes when he finally gets his chance to shine.
I don't think Plummer even liked playing football his last few years. Keenum breathes it.