Just anything really!
I think it's ridiculous that I know more about pregnancy, labour and childbirth NOW than I did BEFORE and WHEN I had my son because NOBODY mentions these things, not even midwives during your antenatals! No one goes over what can potentially happen so you can be a bit more prepared and have a heads up!
I didn't know...
...you could tear muscles.
...you could tear your anal sphincter.
...there was such thing as a 4th degree tear! (just didn't think it was possible - WTF!)
...what 1st, 2nd, 3rd or 4th degree meant - I thought a tear was a tear.
...that episiotomies deliberately cut through muscle and the injury was also INSIDE!
...that episiotomies can cause further tearing!
...you could have other injuries as well as a tear, eg. laceration, graze etc.
...you could tear inside as well!
...that sometimes your stitches come apart and you will be left like it!
...that they don't always suture 1st degree tears - I thought they suture ANY injury!
...that they sometimes do c-sections unnecessarily as it's major surgery that interferes with subsequent pregnancies and births - I thought they considered it a bit more!
...the damage forceps deliveries can cause!
...that they sometimes intervene unnecessarily!
...that not ALL midwives are nice, caring, sympathetic or compassionate.
...that maybe you might NOT be told what injuries you have and what stitches they've done.
...that you wont always have full knowledge of what's going on because they wont tell you. EVER!
...they don't always explain why they intervened!
...that they can sometimes accidentally stitch into your rectum, then have to re-do the stitches! 
...they did an internal exam afterwards and might not tell you what they're doing or why!
...that they don't always ask or wait for consent!
...that they sometimes do a manual removal! (thought you got taken to theatre or wait for retained placenta to come out on it's own! Why is it usually male doctors that do it as well? They have bigger hands!!)
The only reason I knew you bled after having a baby is because when I was about 11 or 12, I saw my sister in hospital just after she had her 2nd. She got up and dripped all over the floor.
Despite the typical stereotypes of babies crying 24/7, you can have an easy-going and quiet baby too that WILL sleep!
I also didn't even think about the fact that wherever you tear or get cut, you will have scars! And that scar tissue will be tight, uncomfortable and annoying, but you're stuck with it (even if you don't know why they cut you in the first place!!)! I guess I didn't think about the biology or physics behind having a baby!
I think that's everything, might think of more, but... anyone else not know any of this stuff? Or is it just me?