My shortest was booked middle of the night 09/19/2014, Arrived the morning of 09/21/2014. Was originally a 8 day trip and we extended it to 11 days by the time we left.
There is a couple things like meal plan and pre-purchasing memory maker that you actually need 3 days prior to arrival which we didn't have. Table Service was initially a problem but once we got the hang of walking up early in the day to see if we could get a time to walk in later in the evening for dinner it got much easier. We also started to discover which restaurants have a while waiting bar, all of which you can purchase a full meal from and never have a reservation. That was plan B when we didn't want quick service yet again. We also discovered that about 11pm up to about midnight a lot of adrs would suddenly be available so we did a lot of next day planning based on where we could get table service for lunch or dinner the night before.
Fastpass we never got the main rides, and the system really wanted to always give us a character greeting. Being an adult only trip, most days we went without fastpass so some child could get the fastpass for their favorite character. I didn't find not having fast passes to be a problem. We started to discover new tricks like my daughter is a late riser and I was getting up late but much earlier than her. I would grab some coffee and go check out the bus lines. We could guess which park was swamped and that would not be our afternoon park. After dinner we would do similar and guess what park was better for evenings. This is when the 1/2 day park strategy really worked out. Afternoon park was when we just wanted to be there and if we could get into anything or see some shows/afternoon parades we were happy. Evening is when we go after the rides and evening entertainment. When needed we would also just change parks if one was really really crowded and we had the luxury of enough time at WDW to do that and not miss anything. I think our longest ride wait was about 15 minutes for 7DMT and we used the get in liine right before the park closes strategy so 10 of it was after the park was closed and no rides were opened for new riders. We also were able to do single lines and took advantage of broken rides when we knew there was more than 1 track and not fully closed down (but the line left).
If your somewhat flexible on where you are, where you eat and jumping around and between parks then last second no time to plan isn't a bad trip. We did and saw everything we wanted plus some extras on this one and will do it again! In our case we like it better, there is a lot of short notice things that many have to keep reworking their plans over and over.
EDITED TO ADD:
Another main downside is at the time there was not many good discounts, this is the trip i'm kicking myself for not getting 1 of us an AP and then seeing what was available and using the limited discounts + getting TIW. We would have saved a lot of money even though park tickets costed more for 1 person.
This year i'm seeing a lot more spontaneous discounts for rooms and things which for us is a great thing! There's more than a few times we almost booked and ran to the airport