Well, the biggest transition is that you have to give up a certain amount of control. Since it is a group tour with a mostly set itinerary, you go where they want you to go, when they want you to go. There *is* usually a certain amount of free time to go back to that cool shop, or hunt out that museum you wanted to go to, or sign up for an off-road tour, but, for the most part, your schedule is set by them. Personally, I'm cool with that in exchange for them taking care of everything for me. In Italy, I've heard people on their own wait for HOURS to get into some of the places (especially in Rome), whereas Disney (and some other tours) have a "fast pass" to get into a much shorter, tour-only line.
They check you in to your hotels, handle all your luggage (it "appears" in your room and is "magically" picked up for the next destination), they arrange for local tour guides/naturalists to take you around & give you tours. They arrange the majority of your meals for you. Disney, in particular, arranges some unique activities you just couldn't arrange on your own.
But if you don't like the hotel, you don't have the option of saying "No thanks, I'll go find another hotel" (unless you have enough money to pay for 2 hotels!)
They provide all your transportation, and water & snacks while in transit & doing activities. If something goes wrong, they will work long and hard to fix it if at all possible.
I travel solo, so I can't speak to the "traveling with other families" portion, but I've read a LOT of reports where people exclaimed over how much all the kids bonded together & had a really fun time as a group. But you're not forced to do that; you can stay as a family unit, also.
Hope this helps a little bit. Personally, I don't think I'll ever go to Europe unless it's with an organized tour like
ABD. It just seems like the easiest, safest way to go.
Sayhello