I agree with what you are saying here as well.
Not trying to find excuses for him because like you, I think he's played pretty good. But I said earlier that it seems to me like he has some lingering lower body issue. Probably nothing major as it hasn't kept him out of the lineup, but like a minor groin or hamstring issue, etc. Watching him specifically, it didn't seem like he had his regular Bogo level skating ability / stride.
Question, wasn't he injured right at the end of TC? What was that injury?
My memory could be off, but didn't Bogosian take a hard hit in one of the first couple/few games? I remember thinking "oh no" and he was on the bench in discomfort. Perhaps that has something to do with it.
Other than that, I think Bogosian's more "conservative" play this season to date stems from playing the left side. For a RH shot and defenceman, it's a pretty big adjustment given your angles are now completely different in how you play the puck, attack an oncoming opposing player or player down low, etc, your mind has to work in a different manner; see Bogo forgetting the other night that he was on the left side and went to play it as if he was on the right side and corrected himself.
I also think this is why Zach has been less physical (if he isn't fighting a minor injury of some sort). To me, he's just playing positionally conservative from the left side, trying to go mistake free (as possible) while adapting to the opposite side he is used to. It's like in year one many wanted to see him rush the puck up ice or try and be more offensive, but his gameplan and mentality that season was to keep it simple, take care of his own zone, build his confidence and defensive game. One thing or area can vastly effect the other.
Now, with all that being said, I completely expect Zach to grow into his normal form the more he plays and gets used to the left side. For some the transition from one side to the other may be easy, while for others it may take some getting used to.