It's so hard thinking back that far, and I definitely think this a better question for non-planners, as you aren't as surprised after researching a place for a year before going.
I remember reading the unofficial guide and being surprised by lots of stuff in it, like the need to make ADRs, and the long section containing instructions for how to eat with Cinderella that seemed a bit ridiculous.
I'm from the south, so heat / humidity and cultural differences weren't a big issue for me.
1. The main thing I think that didn't really translate from paper was what you said, the sheer size, both distance wise and the amount of things to do. I remember very early in the trip being completely exhausted. It was just so physically exhausting. We also went for 10 days, but couldn't fit everything in we wanted to do, and ended up adding on an extra day.
2. The other thing I noticed my first trip was that the food wasn't what we were really use to in the south. Now I love it, but back then, my limited southern taste buds were confused by all the strange things on the menus. I feel like we didn't even know what half the foods on the menu were. I remember us having a hard time finding places to eat our first trip. We were also shocked restaurants didn't serve sweet tea. That is still a pet peeve of mine. At least they have real tea at TRex.
A continuation on that is that we were surprised that the children's menus didn't contain more of what we'd consider to be "kid friendly" or "picky eater" choices. I feel like I had this same issue when we took the girls when they were 11. One will eat anything, but the other two are very picky, and I remember ruling out many restaurants like BOG because they wouldn't eat anything there. Typical kids menus at most US restaurants have chicken strips, pizza, corn dogs, hamburgers, peanut butter and jelly, or a simple sandwich. Simple foods. Usually paired with fruit or fries. Many of Disney's restaurants have kids menus far from that. You can find those options, but usually at QS, not TS. Not a complaint, just surprised us as we were used to taking the kids out to nicer restaurants at home where the adults would eat one thing (maybe French food), but the kids could still order more typical American fare off the kids menu that was more familiar to them. Always seemed strange to me that a place for kids, didn't cater more to what the typical American kid likes to eat.
I did read the menus ahead of time, so we were fine, but there were a number of places I'd have loved to go, if only I could have found something on the menu for the kids to eat. Also, I felt bad because I felt like even when we did find choices they were familiar with, it was the same one or two foods over and over again. They were really sick of the same thing by the end of the trip and thrilled to get back home to "normal" restaurants as they call them. Buffets were great. Tusker House is one of our favorites, and they love trying all the different dishes. So it's not that they won't try new foods, it is just hard to convince them to order foods they aren't familiar with for their entire meal, and expensive if they end up not liking. I'd rather introduce them to new foods at home where it is cheaper.
3. The last thing is that even though we thoroughly researched and planned out everything, things didn't always go as planned. Attractions we thought we would love, we disliked. Ones we thought we wouldn't care for much, we loved. Attractions we thought would be scary to the kids, weren't. Ones we thought wouldn't scare them at all, did. The hour we just knew we needed to set aside for Attraction A, only took 5 minutes, but attraction B, ended up taking 2 hours.