I was there yesterday... I thought that Disneyland in particular felt very empty. The crowd to watch the early parade was very light (I was one deep on the curb and I walked up about ten minutes before it started), and navigating the walkways (at least until those hours before the fireworks) felt positively leisurely.
Where it was PACKED was by Small World before and during Mariah Carey's performance... They were roping off the crowd by segments, allowing small groups of people to move forward at a time, and each time they did even more people crushed together. It was a bit unfortunate, because the closeness of the crowd (combined with people holding up their phones to record) made it pretty much impossible to see the performance unless you were tall or could find just the right angle.
It was still a lot of fun though! She ran through her performance three times, first without fireworks and then two more times with fireworks shooting off behind the Small World facade. Then they ran just the audio of the performance one more time, while they filmed the crowd dancing and clapping along.
A cast member told me that they were also going to be filming another musical guest tonight, but that they didn't know who. Maybe someone there today can fill us in?