Here's what I think, and you'll have to excuse for me for being philosophical but I get like that sometimes.
To me it's a little strange how so many people, including people whose baseball knowledge I totally respect, are so easily dismissive of Papelbon's contributions. He's arguably the best at his position in team history.
I think to a certain degree it reflects the new mentality of baseball fans. We're in an age where salaries are insane, fantasy gaming is huge, and fans get a chance to come on forums like this and play general manager, or fanager, on a daily basis. When a player like Papelbon is let go, the fanagers don't look at it from a baseball perspective as much as they once did. They look at it as 'How can we replace this guy and at what cost? Hey, we can spend that money better elsewhere. Good riddance!'
I've been a fan since the late Sixties. In the late Seventies we had some great teams and then we let some great players like Fisk and Lynn go, and there was outrage that still exists to this day. Hardly any of us who were outraged were thinking about the money saved by letting those players go.
Another thing that works against Papelbon is his personality, I think. Not the most likable guy, said some really stupid things, and seems to be a me first guy all the way. I think that adds to some bias against him.