This is sort of along the same lines as the "What you wish you knew" discussion in Pregnancy & Birth.
Often, we are presented with one image of motherhood, yet our reality turns out to be quite different, for better & worse.
I never realized how hard it is sometimes to get ANYTHING accomplished with a baby/toddler. Just going to the store to get one thing can take 20+ minutes of preparation. There is no "I think I'll go out to eat" and just getting up and leaving, it's trying to find a sitter a few days in advance or taking the kid along and having it be this huge production (did I pack enough diapers, are the people I'll be dining with kid-friendly, the baby spit up on his only outfit, the toddler is bored and acting up before the appetizers even arrive, etc)
I didn't know how utterly exhausted I would be after so many nights of broken sleep.
I didn't realize how much judgment I would get from other mothers. I was judged for circumcising/formula feeding/owning a stroller from the hardcore AP community, and for not letting my baby cry it out/not owning a minivan/giving my child herbal remedies sometimes from the mainstream parenting community. Society at large can be judgmental, but unfortunately, so can other mothers.
I didn't know that there is no such thing as "perfect" parenting, or "perfect" children. Before I had a kid, I thought my child would never have temper tantrums, ask for ice cream for dinner instead of veggies, have a pacifier at bedtime until he was 3, or watch much TV at all. Ha!