IMHO, I believe it all has to do with when you got there and if you had a plan. For instance, our bus on NYE was the first to arrive to Magic Kingdom. It was dark and empty when we arrived. We were some of the first people at the entrance and held back by the rope. And even though a lot of people came and were behind us as it got closer to 7, since we were at the very front it didn't feel that crowded. I bet, though, that those that got there sometime afterward and saw the crowd ahead of them, probably felt it was more crowded than we did. But for us, it didn't feel that way.
When we went in, we walked behind the rope all the way through the castle and to Fantasy Land. From there we walked right onto Peter Pan's Flight and then walked right onto It's a Small World. Afterwards, we thought about going to Big Thunder Mountain but that part was still closed until 8am. We opted to go to Tomorrowland instead. As we went by 7DMT, we saw that the stand-by line was already at 60mins! Yikes! Good thing we had a fp+ for later on that day so we went to Tomorrowland Speedway and waited a few minutes but that was mostly due to waiting for the CMs to add more cars. From there we went to Buzz Lightyear, the wait was about 5 mins. By this time it was just after 8am, we decided to start walking to Adventure Land, still no real crowds. As we crossed the castle we asked one of the photographers to take us a picture, the background looks empty in our picture except for 2 people that had already set up camp (crazy!). From there we went straight to Big Thunder Mountain, there was a line out the door (but I think it was because it was not zig zagging before the stand-by sign) and the posted time read 30 mins, and next door the posted time for Splash Mountain was 5 mins. DH said he would stand in line while I took the kids to Splash Mountain. Splash Mountain turned out to be walk on. When we came out, DH had gotten back in line at Big Thunder because it had gone really fast and he had to ride it by himself. We waited, the posted time was still 30 mins but it only took 15 mins (if that). After Big Thunder the Splash Mountain sign still showed 5 mins, we went for it and walked right on again.
Then we walked to Pirates of the Caribbean and the posted time was 45 mins. We decided to take a coffee break at this point at Tortuga Tavern. Then we waited at POTC for approximately 35-40 mins even though the wait time was supposed to be longer (after our coffee break it had gone from 45mins to 60mins). From there we shopped and had some tags personalized to take back home. At this point it was looking very crowded (but not too bad). We shopped and walked around until it was time for our reservation at Crystal Palace (11:50am). The people ahead of us were turned away and we overheard them turn away those behind us as well. Now, if that had been us, without a reservation, then we would have felt the weight of the crowd more and felt like it was insane to be there at this point. However, for us, since we did have a reservation we were able to go inside and have a nice sit down lunch that lasted almost 2 hours (our waitress was so cool and chatty).
After lunch we walked to the Mad Tea Party (waited about 20-25mins), then we headed towards our first fp+ for the Haunted Mansion. Yes, we were all over the park but we were excited and didn't feel tired at all. We then made our way to our 2nd fp+ at 7DMT, after that we walked to get a Dole Whip and sat for about 30 mins to enjoy it, then we picked up our personalized tags from the POTC store and went back towards Fantasy Land and waited to take a picture with the carriage that had the 2015 sign. From there we went to Space Mountain to use the extra fp that a CM had given us earlier that day. After that, we checked in to our reservations at Be Our Guest. We then went and used our fp+ at Space Mountain, then went to Stitch's Great Escape. We had planned to do the Monsters Inc Laugh Factory but time was going by so fast by this time that it was time to check in to our Dessert Party. Our check in was supposed to be at 11:05pm but we checked in at 10:50pm. The Desserts were meh but the view and reserved chairs/table were totally worth the money and the perfect way to ring in the New Year. My kids couldn't thank me enough for all the reservations through-out our day/night. Through out the day we only felt the crowds thick when CMs were directing us to go all a certain way, as the day wore on and during/after a parade. When we left there we felt like we did so much and had an amazing time. But, had we not had any of the reservations, this would have changed our view of the crowds. We would have been frustrated to wait in line to get something to eat or go to the restrooms (we tried whenever possible to use them during our reservations at the restaurants) and we would have probably said it was a horrible trip. Also, if we had arrived any later than we did, we wouldn't have been able to do as much as we did in the morning and this too might have changed how we felt. But due to how our day/night worked out, the crowds didn't bother us much and we went home (exhausted but happy) remembering what an amazing day/night we had and thinking about a return trip. Also, please note that my children are 9 and 12, had they been younger then they might not have lasted as long as they did, especially with all the crazy walking in the opposite direction of where we would go next.
The reason I posted this message was to let everyone else know how plans and timing (like rope-drop) made a huge difference in how the crowd felt on our visit to the MK on NYE. When I booked my trip back in April 2014, I was so excited that I did as much research as I could. That's how I found the disboards and learned so much about ADRs and FP+. It was here that I learned about the Dessert Party. And I learned from pictures and posts that it would be insanely crowded (beware of the pictures that are posted though, it seems to me that those that post pics of that day specifically choose the most congested parts to snap a picture of). Had I not known about any of these things, I would be cursing and saying how awful our trip was....so THANKS TO ALL THAT CONTRIBUTE TO THESE DISBOARDS. And I hope that by posting my experience, that it can help others that are looking into braving the NYE crowds at MK.