This is not to suggest that un "unplanned trip" is impossible or inadvisable, because, of course it can be done. But it is important to keep in mind the critical difference between "then" and "now", at least as it relates to FP+. In the past, the uber planner and non-planner who showed up at the park at the same time had exactly the same pre-booked adventures--which is to say, none. The uber planner might have had a better idea of what she was going to do first, or the order that she wanted to tour the park. But if she showed up at the Space Mountain FP machines at 10:00 and the non-planner showed up at the same machines at the same time, they each got the same reward. Pre-planning didn't get the uber planner anything that the non-planner couldn't get. Equal footing, if not equal agendas.
But now, with soooooooo many more people utilizing FP+, when the non-planner shows up at the park, the uber planners have already siphoned off a lot of the FPs for the day and since the numbers are finite, once something is gone, it can't come back. The non-planner is no longer on equal footing. That is not to say that the non-planner will get shut out or be unable to do certain things. It is just that their options may be more limited. As long as one goes in with eyes wide open and understands the ramifications, the trip can work out fine. So, for example, if in the past you went with no pre-planning and no agenda, you might have decided upon waking up that today would be a good day to go to DHS. You showed up at 9:30 and decided to get a FP for TSMM. You got in line, and by 10:30 you had a FP for 6:00 p.m. And if you decided not to get a FP and instead got in the Standby line, you waited 75 minutes to ride. Flash forward to today. You wake up and decide at the last minute to go to DHS. You arrive at 9:30 and decide to get a FP for TSMM. You use your phone or kiosk to book one, and you might find that they are gone for the day. You decide to ride Standby, and you will wait the same 75 minutes as before. (At other rides, however, you may find that Standby lines are longer than you remember. So if you were an "I never used FP" person, you might find that your time waiting in lines is longer). You will still get to ride, but your options are more limited. This is simply the reality of the pre-booking FP+ option. If you do not avail yourself of that feature, your options are going to be more limited. Limited to the point of utter disappointment and frustration? Probably not. But only you can determine how much of either you are willing to put up with on your vacation.
As for ADRs, nothing has changed.