I don't want the visual clutter of having other stuff behind the message I'm writing, and I want to use the full screen, or close to it. The new compose forced me to "pop out" the window and maximize it, leaving compose to be 3 operations.

As several people have pointed out there's actually a keyboard shortcut that does what I want ("d") I didn't know about, so now I'm at least slightly less grumpy about it.