Awful color palettes and far too many shades of each hue. If the bluetooth icon is lit but the containing button is unlit, is bluetooth active or not? The icons are some of the worst ever; just a monochrome abstract mess. Of the 12 shown in the quick settings example, I cannot recognize 4 of them. Blurred background in the notification shade is worse than both a solid color or a fully transparent background. The former emphasizes privacy and information clarity; the latter doesn't interrupt whatever you're doing. Blurred means you can't refer to the app screen while looking at a notification but nosey passersby (or clever application of the right deconvolution on a screenshot) can get a general sense of what you were doing.
Too much padding. Alignment is often not thoughtful. Many of the shapes are just strange. The use of different border radii on adjacent elements---notably at 1:09, but also nearly everywhere else---is like a lazy eye or snaggletooth. The "Clear all" notifications button is too big (i.e. easy to accidentally touch) but also an uncomfortable aspect ratio. This is significantly worse on the watch.
Animations totally unnecessary and often not visually intuitive. (Wtf is that jellyfish circle progress meter at timestamp 1:03?) Not every user wants a "cascading droplet effect" or "springy animation" or "happy moments". It's visually noisy. Shape morphing is sea sickness via OLED. It's the kid who can't stop tapping a pencil on a desk and loudly coughing up phlegm all day.
Font is informal and overused. The variable font has more than 10 axes and becomes unreadable at the extremes.
I predict there will be two camps: the developers who don't change their apps at all and those that lean too heavily into this ridiculousness. There will be virtually no tasteful Material Expressive apps.
The promise is "customizable", but it's not USER customizable. You're at the mercy of each app developer. You can't change those awful notification buttons, overly rounded dialogs, bubble images EVERYWHERE.
And it's always so ... busy. I want my most used portable appliance---my vessel of knowledge, communication, and productivity---to just stay still and quiet and do the tasks I request without trying to affect my emotional state and distract me with its unnecessary wiggling and weaving, less identifiable icons, inefficient presentation of info, and irritating nested clickable elements.
Complaining about it is pissing into the wind. Google has committed far too much in this newest terrible design evolution to try something wildly different and actually bold (a la Guardians of the Galaxy) or to return to a simple & timeless no-frills appearance, and there are plenty of users who can't get enough of the "Easter-colored cooked pasta taped to a leaf blower nozzle" aesthetic.
In other words: I don't like it.
