They're a rush based offense. Their PP is more like Carolina's in terms of concept, but they're way better in execution. Makar is a phenomenal PP QB and really the personnel is just... superior. That top unit is amazing.
In terms of how to defend them? I actually thought that after game 1, the Blues did a pretty good job slowing them down 5v5 by walling them up in the nz and forcing more dump ins. It came at the expense of their own offense, but they clearly found something that worked pretty well. Even though they lost 3 out of 5 from that point on, they were in those games and had they focused on hockey instead of killing Kadri in game 4, they may have ended up coming back and winning that one as well.
Makar was generally invisible (and honestly, pretty f***ing poor) in this series for a reason. If you let him do his thing, he's going to kill you. They didn't let him do his usual waltz through the NZ and when he had the puck in his own end, they did a really good job getting in his face. He turned over the puck a bunch in that series.
OTOH, ROR absolutely ate up Mack. We don't have anyone capable of doing that.