The Bruins are just a year removed from a Stanley Cup victory, and finished second in the East in the 2011-2012 season before a first round fizzle-out. They should be a team ready to go.
The Bruins over the past 3 seasons have seemed to fill voids from within while creating new ones at the same time. In 09-10, they couldn't score. In 10-11, they figured out how to score, but the power play was non existent. In 11-12, they still couldn't figure out the power play, but their offense got even better with Tyler Seguin beginning is reign as Boston's superstar.
Entering the 12-13 season it would be expected that with the return of Horton, as well as the progression of Seguin and entrance of Hamilton that the power play struggles would be fixed and the team would improve. If anything, the 2011 season was an indication that the team, although it can have great chemistry, is prone to periods of complete inconsistency both on the front end and back end as well as in net.
Now, usually I'm okay with Chia keeping the core intact, and as for depth its still there. How much difference can you expect from the same team though? Even with Seguin progressing, there isn't much to be gained, even from Horton returning. The problem of inconsistency is still going to be present. I really don't think the team needs anything huge. But there is no spark plug.
The Bruins are in line to once again be a contender, but barring a surprising trade/signing, a first round fizzle wouldn't surprise me this year (not that it did last year, as an invisible top line of Lucic and Krejci are a definite sign of failure.)