I think a lot of it is the combination of the injuries and coaching, but he's had one of the most bizarre career trajectories.
- Enters the league as a relative unknown 18 year old and immediately ascends up the Bruins depth chart at a rapid pace to the point where they feel comfortable trading away Thornton to make Bergeron the focal point up front, and the focus of the Bruins offensive attack. He always had those defensive abilities and crazy hockey sense, but the expectation was always more of an offensive impact than a defensive impact.
- Concussion years where he works away at rebuilding his game, and clearly puts more focus on defense. Takes a back seat offensively with the signing of Savard and the unexpected emergence of Krejci.
- Right around 2010 and the Cup run he started to become the consistent and dominant all-around force that he'd remain for years and years. "One of the most underrated players in the league, amazing defensively and underrated offensively. He could put up more points but he's utilized defensively and puts all of his focus into that." Shows some flashes that he could be more than he currently is with some changes in his PP utilization and the focus on his one-timer. He's not so much a playmaker anymore, he's becoming more of a goal-scorer...
- Then right around the time when Julien was trying to save his ass (yes, even before Cassidy), and right around the time where he started to get over his most debilitating injury to play through since the 2013 Cup Finals, he starts getting used offensively much more with guys that are thinking offense more, the leash is off for all those D-zone starts, and he becomes more than a point per game player. All of this taking place in his early-to-mid 30s. People aren't supposed to all of a sudden round out and find a nice little niche as an elite offensive top-line player at this point, but that's exactly what he's doing, all while sacrificing nothing defensively (well, beside those sweet, sweet defensive zone starts).