The point of icons is to be able to quickly identify something—a button, an application, whatever. That's why people tend not to like changes to them—they're used to the current one, so changes make the icon less useful, at least for a while, and the more drastic the change the worse it is. Changes to a bunch of icons all at once are even worse, obviously.

Ditto other UI changes, like changes in menu or button layouts. People rely on things staying in the same place for speed and confidence using a UI. When they change, you slow them down and make them less confident. Of course people don't like that! Can you imagine the rage if every release of Ubuntu made the kinds of drastic, wide-ranging changes to various command line interfaces that operating systems and major GUI programs do routinely? I think it'd only take about two releases like that to kill the OS, market-wise, even if they managed to do it without breaking scripts and automation (to remove that difference) and even if there were some small benefits to the changes. But when GUI users complain about that stuff people get dismissive and roll their eyes.

Or to further extend that, I've not seen any people like visual changes. Even though the science is there that the new design is better, people don't like change.