There may not be a harder Bruin to grade than Dennis Seidenberg, in no small part because only Tomas Kaberle had a wider gulf between his statistical playoff performance and his perceived playoff performance. The perception of Seidenberg was that he was fairly steady during the regular season, but couldn't quite carry his own pairing, and turned into an ice-time eating, opposing forward-swallowing monster in the playoffs.
The first part of that analysis is correct. The second couldn't be farther from the truth.
In the regular season, Seidenberg posted a respectable, but not stellar, 5.3 GVT. 32 points and a +3 rating were further evidence of rather pedestrian regular season play. When it comes to the postseason, Seidenberg's metric for relative quality of teammates sticks out like a sore thumb: by far the highest on the Boston defense. This is probably due to about 68% of his ice time being shared with Zdeno Chara. You knew that Seidenberg was Chara's partner in the playoffs, but by isolating their performances statistically, the gap between them becomes obvious: Chara had an 8.4 Corsi, Seidenberg -3.5. (Read about Corsi here. At it's most basic, it's a measurement of shots taken and shots allowed.)
The Bruins were a +1.85 per 60 minutes with Chara on the ice, at even strength, +1.16 without him. Compare that to a split of +1.18/+1.46 for Seidenberg; the Bruins were a better team with Seidenberg off the ice. Simply put: for the 32% of the time Seidenberg was on the ice without Chara, he was lost.
Be that as it may, Seidenberg gets high marks for being arguably Boston's best penalty killing defenseman, and for being an incredibly durable player who soaked up ice time like a sponge. That has a value all its own; for every minute that Seidenberg was playing with Chara, the Bruins weren't playing Russian Roulette with the Ference/Boychuk pair.
The 2011 playoffs made it more clear than ever that Dennis Seidenberg is not capable of carrying his own defense pair: like so many others, when taken away from the safety of Zdeno Chara, he falters. The best that can be said for Seidenberg's playoff performance is that he had a great situation, and he took advantage of it.