If you want to go back to the old interface, there's a Chrome extension that does this: Retro Compose for Gmail. I rarely recommend third-party apps that change Gmail, especially if they're not from a reliable company or developer. I checked the extension's source code and I was surprised to see how it works: it changes your browser's user-agent to IE8's user-agent, but only when loading Gmail. As Google says, "you won't be able to try the new compose experience if you're using Internet Explorer 8". I'm not sure why the extension is not restricted to mail.google.com.
I tried to manually change the user-agent in Chrome and Firefox, but Gmail redirected me to this page:
"Gmail requires ActiveX controls to be enabled". Pressing F12 in IE10 and switching to "Browser mode: IE8" worked.
Obviously, this is a temporary workaround. At some point, Google will stop supporting Internet Explorer 8 and this will no longer work. It's a better idea to get used to the new interface.
Update: Here's a similar Chrome extension that doesn't have a button and has better permissions.
{ Thanks, 翔美咲. }