There is one thing that I don't like and what is affecting my decision to return currently. (I currently, for the first time in more than 10 years, do not have a trip planned to Disney World in the next 5 years and am not sure I will add one in). The one thing is the limit to the fastpasses. Our usual touring plan involved utilizing fast pass heavily and staying out of lines as much as possible to enjoy other parts of the park at a more leisurely pace and to keep our sons from complaining about lines, etc.
The limit of 3 per day appears to be designed to balance attraction capacities with guest expectations. Every Disney attraction has a limit to the number of guests it can accommodate in a given day. But the more FastPasses they allow to be pre-booked, the quicker each attraction will "fill."
Disney wants the system to retain flexibility--and really that's what guests want, too. Problem is, if they give every guest 4, 5 or 6 FastPasses per day, suddenly Space Mountain is "fully booked" weeks ahead of time.
Now, since we can only get one headliner fastpass and 2 for minor rides...
Note that this "tiering" of attractions is only being utilized at Hollywood Studios (where TSM has VERY limited daily capacity) and Epcot (where Soarin and Test Track simply cannot accommodate every guest, every day.) At Magic Kingdom and Animal Kingdom you can pick any 3 attractions.
...it seems we'll be spending almost all our time in line and we'll end up not being able to ride as many rides. We used to have up to 7 fastpasses in one day (same park) and it greatly enhanced our experience in the park. Being able to cut the wait at only one ride where it really matters tells me the next trip would be a grueling gauntlet of going from line to line.
In many ways, it's a trade off. If you really are getting 7 FastPasses per day, you're spending a lot of time going out-of-your-way to first obtain the paper FP tickets...then killing time waiting for your ride time to arrive...and finally returning to the attraction to ride.
While the legacy FP system did lead to shorter in-line wait times, all other aspects of it were very inefficient. On the days when we did do our best to maximize FP usage, we always ended up criss-crossing the park multiple times, splitting up our party so that someone could run and get FPs, and we were constantly playing the "what do we do now" game in between those uncertain FP return times.
FastPass+ largely removes those inefficiencies. A seasoned Disney park guest knows about what time they arrive at the parks, the order in which they prefer to tour, and the best attractions on which to use FastPass.
Yes, you may find yourself forced to spend 15 minutes in line for Buzz Lightyear if you don't want to spend a FP on that. But is that any worse than finishing all of the Tomorrowland attractions 20 minutes before your Space Mountain FP ticketed return time and then saying "OK, how do we kill the next 20 minutes?"
An individual affiliated with one of the websites that does custom, computerized WDW touring plans (don't know if I can mention the name) recently concluded that most people would come out ahead with FastPass+.
Those of the "commando" ilk merely need to make sure they're in the park at rope drop. When visiting Epcot, get a FP+ for Soarin and go immediately to Test Track after opening when there is no Standby line. Same approach for Hollywood Studios and attractions like RnR and TSM; do one at opening and FP+ the other.
The scenario which will be most difficult to replicate under FP+ is riding the same attraction over and over again. With the legacy FP, you could get 3 or 4 FPs per day for Space Mountain if you wish. That's no longer possible. But I also believe it's by design--Disney's attempt to make each attraction accessible to more groups at the expense of the heavy repeat users.