Thomas is an extremely talented goalie but he's not the only one on that team. Seguin wasn't named NHL's first star recently for no reason, he's really been stepping up this year. He learned a lot from last year and it clearly shows, he's made some amazing shots. (Note both players were amazing particularly in the B's win against the Leafs recently). Chara, no doubt, also very talented (doesn't hurt that he's enormous...). Marchand, Bergeron, Horton, Seidenberg, Lucic, Paille, Thornton, all very good players. Obviously other teams have good players too, those are just the ones of the Bruins who I find particularly admirable. They weren't playing too well at the start of the season (inconsistency, in particular), but they've been playing a lot better recently (note that what, 7 game winning streak?). Julien has spoken in particular about that.
Just as a disclaimer, not saying the Bruins are "like, OMG, the best, like EVER! OMGGGG". But they're definitely a strong team.