I'd second many of the ones above and add these:
That you may be told that your pain relief is interfering with your pushing and be expected to do the most painful bit at the end of labour without it!
That the staff dealing with you are unlikely to hold much truck with hypnobirthing.
That you are unlikely to get away with 'breathing down' instead of pushing.
That it is well nigh impossible to have a say, discuss things, let your wishes be known or decide things as you go along in labour because you are so overwhelmed by unbearable gut-wrenching pain (the like of which you have never experienced and never believed existed) that you will be lucky to have a coherent thought, let alone weigh your options or string a sentence together-and if you do no one will be interested).
That labour as it is conducted by traditional medical types (most of the staff you are likely to come across on a maternity ward) is some kind of throwback medieval practice worse than a cross between being given an operation without anaesthetic and getting old fashioned psychiatric treatment and they get away with perpetuating it because all the patients are women who have fallen for taking it as a badge of honour / right of passage /sign of their love to suffer as much as possible for their babies in ways men wouldn't even consider for a minute! Blinding people with science and scary, one-sided, risk-based justifications of their practices also helps them to convince us. After the event they are buffered from any comeback by our sheer joy and gratitude that our babies are there, alive and in tact in the end if, thank god, they are, as well as us being otherwise occupied with baby, possibly traumatised, almost certainly sleep deprived and dealing with lingering pain, mutilation, disfigurement, loss of function and all the various delights mentioned above.
That the maternity trade transports you back to the 1950s and is populated by people who wanted to be evil nuns but were a few decades too late to catch the heyday.
That many of the midwives and doctors in the field who aren't quite psychopaths have either had empathy bypasses or were never blessed with any, that they will often not even pretend to care about your illness or pain, that you will repeatedly be asked how you are and then be told that nothing can be done about it with a Gallic shrug (so why ask then???), that being concerned about your own welfare in any way will be greeted by disapproving or dismissive glances and throw away comments or patronising lectures on putting your baby first, that there will be a student at almost every consultation and you will not usually be asked your consent for this, that midwives may well prod you and explain to their student what they are doing whilst not acknowledging your existence or the pain they are causing you.
That nobody is remotely interested in your 'birth plan' and that nobody will even read it, that all the things that you agonise about that are supposed to require your decision or consent will be decided for you without your consent, that staff will be shocked if you do try to make a decision about your or your baby's treatment, that with all the best intentions your partner will soon be scared into submission if they try to enforce your birth plan for you.
That aftercare will be more like being jettisoned and that if you do ring the triage in the dead of night when you are worried about your baby's welfare they are likely to actually take the p* out of you and leave you with a choice comment like 'welcome to parenthood' ringing in your ear!
That you can know and experience all this and still not care because of the infinite love, joy, fulfilment and wellbeing that your little one has brought flooding into your life :)
(not forgetting those who can not feel like this at all and can come down with PND or PTSD. I feel blessed to have dodged those bullets and all sympathy to those who haven't)
Happy motherhood everyone xxx