I understand about the additional 8 hours of idle time. The Watch sat on my desk right next to my phone all night. I have had it last two full days several times in the past so it drained faster than I have seen previously.
TxWatch
given the watch isn't intended to be left off charge and not worn ("all day" battery life and an emphasis on the nightstand show apple clearly wants you to put it on getting out of bed and take it off when you get into bed) - i wouldn't be surprised if the watch does *more* (checking in with the phone, etc.) when it's idle off the wrist than when it's on it.