* The name: What do a bunch of geometric, flat-looking, solid colors have anything to do with "materials." "Material Design" always makes me think the direct opposite: skeuomorphism.
* The look: It goes way past "clean" and "elegant" into "bland" and "childish." I don't want a bunch of tiny buttons with indecipherable icons and pastel colors. I want well-organized controls with obvious labeling and a consistent color scheme.
* The appeal: sites like this seem to be encouraging general usage, but then your site just looks like you ripped off Google's stylesheet. (And like I mentioned above, I don't know why you'd want to go for that look unless required.)
While I'm griping: it also drives me nuts that Google insists on forcing MD where it doesn't belong. They refuse to follow conventions on iOS so things like the navbar and the "current location" or "share icon" don't match any other app on the device.