Everyone says Baby Z is "a good baby." This irritates me a bit. "Good" seems to be defined by "not a lot of trouble" rather than "nice." Baby Z is extremely sweet-tempered and social, only cries when necessary, and even then is really low-key and discreet. But even if he weren't this way, I would still love him as much as I do now, regardless of the adjective. (Granted, my view is overshadowed by the whole breastfeeding thing, but still.)
the good baby doesn't complain about the car seat
I usually just think of 'good' being the opposite of 'fussy' or 'colicky' - I've had friends that report that their children were just 'difficult' children until they were a toddler. I was blessed with 'good' children too... but I never say that. I just tell people that my babies were pleasant.
Baby Z looks like he's a JOY though. That grin is infectious. What a handsome little fella!
Typically when I say a baby is good, it's because they're not fussy. I don't know what to do with fussy babies, and babies who cry make me nervous and anxious as a result. When I use good to describe toddlers and older kids, it's more that they're nice kids (like, they share and don't compete for attention with other kids around because that usually ends with them being mean to the other kids).
Baby Z is so adorable. This picture is amazing!