Everything that has been said so far is so true. From my observations (that are similar to what previous posters said with a few variations):
Workload: Wow is it a lot of work. And most med students are smart enough to have done their research going in to know that there's going to be a lot of work and a lot of stress. But no one KNOWS knows really how bad it is until they get there. Even during weeks when you think the information isn't that much and you have kind of a laidback week because you think you have the info on lockdown, you get a quiz back and you're like what the eff???
Obligations: There's so much administrative bullhonky they throw in your face everyday you wonder if the administrators are just complete sadists. Granted a lot of what is required of them to require of us is driven by LCME, AAMC, [insert other acronymic institutions], but they sometimes seem to go overboard. And they don't always have a schedule of things beforehand so you have your week planned out and then you get an email blast that's URGENT and there's some mandatory crap you have to do that takes over an hour that you didn't schedule for. The other obligations are kind of self-imposed but obligations nonetheless. You'll be signing up for clubs left and right so you can have a great CV and then you have keep up with them AND your grades AND your relationships AND your health.
Identity formation: Your 20s are the time to really figure out who you are. Luckily I started med school in my 30s and I went through all of those major identity crises before starting med school (but tbh I still have crises so that never really goes away). Even if you don't have the conscious realization that "hey, I don't think I know who I am," the thoughts are brewing underneath. You do a lot of growing up in your 20s but since those of you in your 20s are in med school, you don't even have time to reflect on who you really are and what you really want out of life. You're lucky if you leave the house having brushed your teeth. I would relate med school in your 20s to early closure of epiphyseal growth plates due to extreme stress: because of med school, you'll be a little stunted emotionally/mentally. Hopefully you'll be able to change that after residency, but who knows. I honestly don't know how anyone does med school before 24. Y'all are a lot more mature than I probably was.
Reality check: This kind of goes with identity formation, but I think it deserves a separate short paragraph. Like
@Osteoth said, reality and expectations definitely don't match up. You think you're gonna kickass in med school and stand out and save lives but, more than likely, you'll be in the middle or below, be just another face in the crowd, and not do much doctor-y type things for a very long time.
Keeping up appearances: So everyone jokes around about being stressed and not having time to do laundry, etc. etc. but very few people really open up about the current state of their mental health. It's for several reasons, but the major two are 1) no one wants to appear weak in front of their peers and future colleagues and 2) fear of these things being on your record. For 1) everyone is trying to keep up a good front. No matter how much it's drilled into us that it's ok to talk to a counselor or to admit you're depressed or so stressed that your hair's falling out, who's going to speak up if no one else is? Who is going to go to someone and cry on their shoulders? Even if it does happen, everyone keeps it on the d-l so the cycle of silence happens again. And this is just within med school. You also worry about keeping up appearances for non-med folk. How can you tell your non-medical family and friends that school is causing you major depression? They all know med school is hard, but to them you're living the dream, you get to be a doctor, what's there to cry about? So even if you do have sympathetic family/friends, you feel like a spoiled brat for even thinking about opening up to them because maybe they didn't get to live their dreams, but you're doing it now. For 2) even if you were brave enough to admit these things, it might go on your record somewhere. It probably won't go on your transcript or dean's letter, but it might show up on some official thing somewhere else. The school will say it won't, but can you really trust anyone? Even if the school doesn't put anything like this on your record, you still have to worry about state licensing and the questions you might have to answer there.
Sorry for the weirdly long post, but I felt it was important to get all of that out there. Some of these things I've experienced and some are just observations. I hope this helps someone some day. Please learn from our collective mistakes.
Great thread.