Garish distracting colors where the old one used muted colors that were easy on the eye.
The new Google calendar app pushed to Android within the last few weeks is a perfect example of this. In the old one, I could easily pick out dates and items. In the new one, the colors overwhelm everything and I literally have to consciously focus my mind to "see" the data they are presenting. It's a complete FAIL, IMO. I wish there was an option to opt out of the material design version.