See, my experience with the Current FP system isn't like other peoples, our trips have been in February and the first couple weeks of May, and I know crowds were not at their peak.
For us, FPs, were always available for the headliners. There might be an hour or even 2 before FP return, but they were there. Except for toy story at HS
So when people talk about doing "rope drop dash" for FPs, that has not been our experience. We don't show up for rope drop, we aren't morning people while on vacation, we like to sleep, show up at 9 or 10 and go from there.
That's why FP+ excited me at first, "sweet, sleep til NOON" go in for the FPs.
BUT, now I am thinking we have never stuck to our schedule of parks for the trip, something has always come up. This will be the most restrictive aspect you could add to a WDW trip. I literally worry that this will so dramatically change the "feel" of the trip, from a vacation to a scheduling session, that we will no longer enjoy WDW. ADRs don't do this, we are never concerned with what park we are "doing that day" we book where we would like to eat 180 days out, and we get park hoppers so we can always bounce between parks.
Another thing that has come to my mind that could be problematic. Rides like the Mine Train are obviously going to book up insanely fast. I wonder then what you do if you book your FPs for the other rides, get 9am, 10 am, and then your mine train comes back at 5pm.
Now I get that this can happen at the parks, BUT, I think its much less likely to happen at the parks (although as I pointed out, by 2 or 3 pm Toy story FPs were gone on our last trip). However, those people at least had to be at the park, you had until 2 or 3 that day to get a FP (we in fact got out FPs at 10ish and rode TS at noon). With FP+ people wont have to be at the park, they will book these things up completely LONG in advance.
How many FPs will be wasted ? People booking FPs and then not showing up to the park, or deciding their FPs are too inconvenient ie: 9 am 10am and 5pm ?
There will no longer be that pixie dust of some random person walking up to you and saying "Hey, we didn't use these FPs" or "they didn't take our FPs slips" YOU go ahead and enjoy them.
I just don't see how this system adds anything good, at best it will work as well as the current system, but I see many potential downsides.