This was our first NYE at WDW, so I can't compare to previous years. But I can share our experiences.
MK on Dec 30 was very, very busy but manageable. Stayed through NYEE fireworks, towards the front (non-Hub end) of Main Street. People were continuing to arrive at that end of Main Street up until shortly before midnight. It was very dreary and wet - but the fireworks were worth it!!! Up until then, major thoroughfares were very crowded but kept moving.
Epcot on Dec 31 was (by our experience) nuts! We had FP+ for the 6 pm IllumiNations. After the show, foot traffic towards the International Gateway was not moving. As in, crowds were standing, facing France, and not moving. People started jumping the low fences and walking on the grass. Took us more than 30 minutes to get from the FP+ area to the IG. We were scheduled to have dinner at Tutto at 8:35, then see the late fireworks. We chose to bail out and see the late fireworks from the Boardwalk. It was too crowded to be any fun at all for us.
Nice experience - I called to cancel our Tutto reservation, fully expecting to pay the $10/person penalty, but wanting someone else to have the opportunity to use "our" table. Had to be patient as I was worked through the automated system. But it kicked me out to a CM. She asked me why we were cancelling. I told her we had been in Epcot but it was just too crowded for us, but that I wanted someone else to have the opportunity to eat at Tutto. She cancelled our ADR, and also cancelled the penalty. I wouldn't ever count on that happening again, but it was a nice bit of pixie dust that day!