I guess that slowing down Sonic would make him a little easier to control and Iizuka did say he wanted Sonic to appeal just a little more to the Mario crowd, so some changes would be inevitable, the Donkey Kong-ish style of those character models seems to be an indication of that.
But just one second, I can understand why some might dislike Sonic 4, it's a 2D Sonic game entirely built from the ground-up and there seems to be a dash of Sonic Unleashed into it, through the constant use of speed bumps that are there only to propel the player foward, but they serve exactly that purpose.