"Default to full screen". I hate it too. Enough of the boxes and frames. Maybe I am old school but I like pages to flow. Google is getting terrible with their UI's. From a disability access point of view, it is hard to navigate around the new Google "plussed" world. Crappy UI's and user experience combined with their recent views on GMAIL users should not expect privacy, Google is on the outs with me. Everyone is taking off with HTML5 but very few sites, not including Google, is getting it right.
I believe Google switched everyone over to the new compose a few days ago. If you click the little triangle in the bottom left of the compose window, to the right of the garbage can, you'll see an option to default to full screen. I think that's as close as you'll get to the old compose experience.
You can always switch to Basic HTML view. It's dated, but it stays out of the way.