I have no words! It is so wrong on so many levels:

The size

Too small.

The placement

Lower right corner? Seriously? So you feel compelled to bend down to peek at it. My first intuitive motion was to try and grab the window bar an drag that pop-up to the upper portion of the screen, to my eye-level. But no, it would not budge!

Functionality

Lacking! If I wanted a chat program (which I don’t), I’d use one. E-mails are for serious communications.

Maximise

One more click is required to pop this tiny compose out into a…. new browser window?! I don’t want a new browser window! I use tabs!

In stead of conclusion

In a comment by user Chalchihuite here I read a brilliant would-be dialogue between a Google developer an interior decorator, which would demonstrate the thought process (or lack thereof) behind the new Compose mis-feature:

Decorator: …And then we’ll put the couch over here.

Gmail programmer: Oh, our data show that people only sit on living room couches 20% of the time. So it makes more sense to keep the couch in the garage, and tell people to carry it in whenever they feel like watching TV. By the way, we put the TV down in the basement. Look for the box labelled with a mysterious icon.

Decorator: Wouldn’t it be a lot easier to just keep the couch and TV here, so they’re available whenever people feel like using them?

Gmail programmer: Nah, people just have to get used to the new system. (Chuckles) Besides, what are they going to do, move?

Decorator: Well…okay. Was there anything that you wanted to put in place of the items you took out?

Gmail programmer: Ick, no. I want to shrink the room down to a 6′-by-2′-by-1′ box. Rigorous analysis shows that a majority of room users are under 6 feet in height, and less than 2 feet in width. And basic logic indicates that no one requires more space than the volume of one’s own body. It’s just good, efficient design.

Decorator: You realize you’re describing a coffin, right? Besides, what about people who want to sit down and watch TV? Or who are more than 6 feet tall?

Gmail programmer: Geez, what a whiner. Those weirdos can open the trapdoor marked with a mysterious icon and ride the chute down to a convenient subterranean grotto, where they’re free to indulge in perversions like “sitting.” It’ll be easy for them as long as they’re careful not to open the other trapdoor, marked with the other mysterious icon. That leads to a cobra pit.

Decorator: I know you believe that a tiny, bare box will be simpler and cleaner than a large, well-furnished room, but I think you’re actually creating a lot of needless complexity. You’re making it difficult and frustrating for people to do basic things, which they’re already used to doing without any problem at all.

Gmail programmer: DATA! EFFICIENCY! TABLETS! TEXTING! TWITTER! TEENAGERS! CLEAN! CLEAN! CLEEEEEAAAAANNNNN!!!!!!!!