It's tough to put a finger on what exactly is going on. The Blues have generally been activating their defense less, been a bit less physical in puck battles, and been a bit less effective at maintaining puck possession both in transition and into the offensive zone. Slightly bigger gaps on the backcheck, and in the defensive zone coverage. Always a half-step to a step behind when applying pressure, so less effective at dictating play and more chasing.
Nothing huge in any particular category, but a bunch of small differences that all add up in the end. Mostly, I think all that is stemming from slightly more tentative play than we're used to seeing under Hitchcock. We've fallen into the habit of settling for playing the type of game that the other team wants to play, and gotten away from forcing them to play the type of game we want to play. It's a fine line, and an easy one to fall back across when you're a young team.