So much! I think the main things are just being really scheduled and not having the freedom to just do whatever you want, which sounds kind of obviously, but I don't think it was before I became a mother. Like there is rarely a second of time that doesn't have something that needs to be done or that is constraining what we can do. This is something people always seemed to lament about but that I never understood before I had children. Like how can it be that hard to find time to shower? Or what do you mean you haven't had a night out in a year?
We bedshare with our now 2 year old, so literally, the only time we aren't with her is when we're at work or for about 3 hours in the evenings. She's with us all night. She's there first thing in the morning (no time for morning alone time with my husband anymore!). We're busy caring for her and doing breakfast and getting her ready for preschool until we walk out the door at 8:45. We work until 5 or later depending on the night. One of us picks her up at 5 and then we're with her until 8 when she goes to bed. Then we get to sit down in exhaustion and stare at the tv in silence until 10pm before we drag ourselves to bed. I can't spontaneously run out to the store in the evenings like a used to. I have to do my shopping online during the week and have it delivered (we live 30 minutes from a shop). If friends invite us over for dinner randomly, we can't go because we need to book a babysitter like a month in advance. We have no family nearby. You just literally can't leave the house the way you can when you don't have children. If both of us need to work late, one of us has to take off work because someone always has to be at preschool to pick her up at 5. There just isn't flexibility anymore and that was a real struggle. It's also really hard to explain to friends who don't have children and people get really bent out of shape when you can't come to such and such thing at their house at 9pm on a random night. But you can't just leave your children and babysitters aren't cheap. They're an occasional (maybe once a month) luxury. We've left the house after dark maybe 10 times in 2 years. Of course, it may be different if you have family nearby who can help, but no matter how much family you have, you still just lose a degree of flexibility and spontaneity.
Even when you do go out, you are never carefree like you could be when you didn't have children. My husband and I occasionally squeeze in a night of dinner and drinks out. But you literally feel like Cinderella. At 11pm, your babysitter needs to go home and your chariot turns back into a pumpkin. There's no late carefree nights anymore. Even when you do go out, you spend half the time talking about your child because you're not used to talking about anything else. Even when you are away for a night - I've travelled a bit for work since I had my daughter, with my husband at home with her - I don't feel like I can ever truly relax. I'm always thinking about whether he remembered to give her the peas with dinner and if I did enough washing so she had clothes for school while I was away. It's not like a totally, fun, me-time kind of travel. You feel guilty and you're still half at home in your head.
It also puts more strain on your relationship than anything else ever possibly could (except for maybe an affair or a serious life-threatening illness). Even little things that your partner does that never caused any problems before will become magnified 1000x. My husband has never been a morning person. When it was just the two of us, it didn't matter at all. I just got up and read or made myself some tea and let him sleep. When you've had 2 hours of sleep each night for the past 2 months and you're up trying to feed a baby and you didn't even have time to shower the past two days and you may not have even brushed your teeth last night because you didn't have a spare hand while trying to comfort a sick baby, and your lovely husband is snoring away next to you, you will feel resentment and rage like nothing else. You can talk about it over and over and they will still do dumb things like that (you'll do dumb things too, which will no doubt piss him off). You won't have eaten a single meal without a baby attached to you for several months when your lovely husband will tell you he's going away for a weekend with his (childless, generally responsibility-free) friends and leaving you alone without back-up for two nights. You will see red. It will be fine. You'll (usually) work through it. But there is definitely a reason why so many break-ups occur in the first 2 years. I found that after the 2 year point, things have gotten more manageable and we have more time and energy to devote to ourselves as a couple. But the first year or so is really, really hard. Make sure your relationship is a solid as it can possibly be before you head down the road towards parenthood. It will take a beating and you will have days when you hate the sight of each other and you might even consider leaving (this coming from someone who'd had like 2 arguments ever with her husband before this), but you'll make it if the foundation is pretty solid.
I would say, also just the cost of childcare is a huge thing. It's anywhere from half to all of my income every month. Some months I literally work to just pay for childcare. It's so much money. Again, that might not be the case if you have family who can provide some help with childcare, even a few days a week, or if you plan to be at home and you can survive just on the income of your partner. I don't find children to otherwise be expensive. You don't need much in the way of clothes or toys (though people spend a lot of money on these because they want to, they don't need to). But childcare costs alone can be really life-changing if you are used to having a certain income to survive.
That doesn't mean that having a child isn't the most incredibly awesome experience. It is. But you will never have things as easy and straightforward after you become a parent as you did before. Do all the things you want to do for yourself and be selfish now. I didn't have my daughter until I was 32. I did 2 degrees and was part of the way through a third when I had her (which I'll finish next year). I travelled and lived and worked all over the world. I did lots of stupidly foolish things. I partied. I stayed up all night. I slept all day if I wanted. I got a lot of that out of my system. It doesn't mean I don't still wish I could go out drinking and not have to worry about being up with a toddler at 2am or that I don't still wish I could sleep past 6am. I do wish I could still do both of those things occasionally. But I don't feel like I missed out on anything before life totally and completely changed.