A few things come to mind...

(1) Feeding a newborn can take approximately 8 hours out of every day. It's a full-time job just to get your baby fed!

(2) Baby sleep is a hands-on endeavor. It's not just put the baby down, and s/he naps. It often takes continuous rocking, jiggling, strolling, etc. to get baby down and keep her asleep.

(3) Newborns have pretty much zero control over their arms and legs. Their arms tend to just flail around, so they certainly can't grab for a toy or pop their thumb in their mouth purposefully during those early weeks.

These are all pretty important aspects of newborn life that I just didn't get until I became a parent. What things about babies did you not really understand until you had one of your own?