1. Development/physical disabilities, or overall health issues
2. Reaction from others, will immediate/extended family help? It's one thing to ask, another for the reality to set in. Overall support base in general.
3. Employment realities. Will a lot of employers have explicitly stated policies, the day-to-day realities often differ (and can be very different between mum(s)/dad(s)).
4. Just general variations in children/growth. I have two, one eats but doesn't sleep well, the other doesn't eat well but sleeps soundly. One enjoys time to himself, but has a temper. The other is far more social (so much social) but is very even-tempered and empathic. No amount of preparation can provide enough knowledge/experience to deal with these qualities, not withstanding 1/2/3.
As others said, we read books, attended classes, looked after nieces/nephews, younger siblings, baby sat etc. We were still woefully unprepared for our own.