The atrocious farting. When pg. Also after birth. Has never completely gone away tbh. 
How much the whole thing bloody hurt. I expected labour to be bad, knew about the lochia, etc., but never expected to be pretty much unable to walk for weeks and in terrible pain down there for months (and applying bactroban constantly) to my poor ruined perineum, courtesy of what the registrar delicately termed a "complex repair" 
Pooing after birth. You know when they tell you not to worry because you can't burst your stitches even though it might feel like it? Well, that one's not true. Hello first agonising poo after birth; bye bye, part of "complex repair"
Was so traumatised by first poo that the second was even worse. I drank and ate virtually nothing in a hot hospital for a week and ended up in a near "manual extraction" situation. Was taking Lactulose for months.
That even though fully EBF round the clock, my periods could return exactly 4 months to the day after birth, and that the first few would be like someone had turned a tap on, complete with clots the size of golf balls, etc.
That newborns vomit up bright yellow mucous from their lungs for a few days and that this is normal.
That some babies' hair falls out completely and then grows back in in a completely different colour.
That newborn babies in the first few weeks are a bit wonky-looking and look nothing like their future selves. I spent weeks crying because DD had a funny hideous little goblin face, an ear which had folded over during the (forceps) birth, a flat nose and one eye visibly higher than the other. For weeks I mourned my poor unattractive child's future blighted life with her wonky misaligned eyes and folded ear. Four months later she looked nothing like that - and she is now a remarkably beautiful toddler (not just my opinion, but people stopping to comment in the street how lovely she is kind of beautiful). I wish I had not been so upset about it after the birth. I had PND and was generally unsupported, and was having a terrible time generally, but had someone gently said to me that newborns can look a bit wonky at first and that they don't look how they will look later, it would have saved me a lot of worry and misery and made all the difference.