When I use the "image" button, instead of bringing up the usual dialogue box, it now has this monstrosity.
Also, I no longer seem to be able to add copied images so there's a loss of functionality.
Quoting from a question about today’s CSS bugs on Meta.SE:
We rolled out a new image uploader today. Unfortunately, the design was targeted at the new-design sites only, and so the image uploader stopped working (at all) on the old-design sites.
In a hurry to get the image uploader working, we grafted the new image uploader styles to the old base styles. This had some of the side effects you noticed. We have since fixed these side effects. My sincere gratitude to our design team for helping out with this in off-hours!
I now see a very different uploader, which seems to work correctly (I used it to upload this image), and looks very much newer: