Ice Age 2 was already an immediate notable downgrade. The colors were brighter, they're suddenly in a town place with a crazy wackjob theorist and conman, Sid is babysitting some dumb children, that thing with the mini-sloths, the weasel brothers at all, etc. Plus the music while still great, wasn't like the first one which had a very specific feel.https://www.youtube.com/watch?v=vACZwZZKpAwhttps://www.youtube.com/watch?v=3Q4cRMMXQt8
It's weird because the first movie was already going to be a 2D more action-oriented type film and then around the switch to Blue Sky they decided to let it be more comedic, which is why the VAs are who they are, but still ended up a fairly serious and toned down type. The second movie just got more childish and the franchise just kept going like that. Also Diego was going to die in the first one but the test audience took it too hard. They also added in more Scrat (it was originally just the opening scene) for them too, but they did it in a very good way without detracting from the film's quality.
I wish we could have just gotten one sequel and it be in the same mindset of the first one, maybe with their increased animation detail, because Blue Sky's animation did get pretty good as it went.