Those that have genuinely done no planning except found out what hotel they wanted and decided to go to the Magic Kingdom first generally have a bad time of it. They walk in at 11 am having missed the best hours of the day both heat wise and crowd wise and then waste 10 minutes at the RR bridge looking at their map and deciding where to go first. They end up in long lines, tired, hot, thirsty, and frustrated when they pass by the area for that popular princess meal and can't get in. They come away thinking Disney is just not worth it. And what is worse, is some families go having issues. Well unfortunately Disney doesn't solve them. Nor do the issues wait outside the gate. On the contrary, Disney brings these issues right to the surface and lays them bare for all to see. And it's not pretty when it happens. I can't say that arguments, yelling and the like and sometimes even out and out fights are on the rise. I only know that I am seeing more of them.
Now someone like me that knows the place can get away with a week before hand reserving a room somewhere and going on a whim and still have a great time. I can find those places where things open up day of or while we're down there and adjust. Most are not to that point yet.
Not exactly disagreeing, but a lot of it depends on your travel and personality style. The first time we took my son to MK was on a whim. We were at the beach and said hey let's drive up to Disney tomorrow morning. Disclaimer it was Oct so not the insane summer crowd, but we had a fantastic time! We just rolled with it, rode the rides we remembered as children (and back in the day befor FP you had hour long waits so longer lines weren't a huge deal), we didn't eat at any of the really hard to get restaurants but we did do TS without a problem. We strolled, occasionally looked at the map, and it was great. Three years later we did it again but with my mom in tow this time-same day before whim decision.
If you are an uber planner, commando park visitor (and nothing wrong with that) the lack of a plan and possibility of a missed ride or experience would have a bad time of it. However I think those of us with a different approach also have just as great a time of it just with different expectations.