There's so much stuff. Some of it is individual, though. Like a lot of people, I was completely unprepared for the fact that the baby could feed for hours at a time - and of course all the books etc pretend this doesn't happen, so I thought it was just me.
My HV always told me that I shouldn't feed my DD to sleep - that I should always put her in her cot awake. I ignored this advice completely but always felt slightly guilty about it. I recently read another expert (there are so many, aren't there?) who said that feeding the baby to sleep is perfectly fine and normal.
I wish I'd known that not everyone has a let-down reflex - I didn't, and I never really understood what it was.
Finally, and this is possibly peculiar to me, I wish I'd understood about breast abscesses. Early on I had what appeared to be a blocked milk duct that wouldn't go away despite massaging it. I then developed a red spot on my breast which I knew was a sign of mastitis - I expected to develop a fever at which point, I thought I'd go to my GP. I didn't, so I didn't really know what was wrong.
I eventually did go to my GP and it turned out that the blocked duct had become an abscess without going through the intermediate stage of mastitis (all the books will tell you that an abscess only arises from untreated mastitis). I then had the most horrific experience with a variety of hospital doctors who didn't know how to treat a breast abscess. I won't bore you with all this, though, as it's a long and painful story.