I'm one who was able to book FP+'s yesterday, and I had the same reaction when I was booking them. As a super-planner, I spend months nailing down my itinerary, ADR's and touring plans. Hey, it's a hobby!

But, I also know how to "read" the parks and adjust on the go depending on factors like crowd level changes, weather, ride closures, etc., so we are able to have flexible plans It stressed me out more than I thought it would picking the FP+ times, because I kept second-guessing myself. What if that ADR runs long? What if we want to go back to the resort and take a nap after all? What if we don't want another day at EP and want to hop to MK? It absolutely allows for much less spontaneity than the current FP system.
I thought that scheduling HS was the most difficult, because of the shows.
It's like one of those logic problem: "Suzy has an ADR at Sci-Fi at 12:45, and one at 50's Prime Time at 7:00. She also wants to plan for Characterpalooza at 5:30. Her FP+ return times are 1-2, 3-4, and 4-5. What times can Suzy go see Beauty and the Beast and Indiana Jones?"