The idea of a players only list sounds good, but when you think about it, an NFL team only plays 13 different teams in a season. So ultimately if the players are pulled evenly from across all of the NFL teams, over half of the players who are voting on it, never played against your team this year.
So what are they left with? The same stuff the fans are left with. Team wins, reputation, stats. And in fact it may be worse then fans because while fans are busy actually watching the games, the players on the other hand are kind of busy on Sunday playing someone else. So in a sense they may in fact be less informed then the fans.
And that's doesn't even take into account the fact that offensive players don't play against other offensive players.
So if you are looking at players who actually played against a specific player on the field at the same time this year, it's not a very large percentage. Maybe 20 to 30% at most.