When Gmail introduced its new and minimal compose window in October last year, you had the option to revert to the old version if you preferred it. That option will shortly disappear.
The Gmail team confirmed in a Google+ posting that the new Compose option is now set to become a default. One major factor in that decision: the recent introduction of a full-screen option, which was one of the most often-heard complaints about the new approach. The ability to launch in a new tab via a keyboard shortcut also helps.
I'm sure some veteran Gmail users will complain about this change, but that's inevitable whenever anything changes on a product with this many users.