I like the general direction. It gives a very clean first impression.

Some issues:

affordance

There is no indication that is is possible to write something under the subject line.
I am afraid some users will try to fit their whole email in the subject field.
Then again, once the press "Enter" it should all become clear.
Some placeholder text for the body could solve this issue.

minor inconsistency

Once you are in the body ediror, and there is no text,
I would expect "backspace" to get me back to the subject line
(because "Enter" brought me to the body editor)

waste of screen space:

This is probably my biggest concern with this design.
The margins become really big when the window is full size.
What was your reason for having the maximum width fixed?

attachment placement

that's something we might want to look at.
It's not wrong. But we might be able to do better.

All in all this is awesome. It just needs some polishing.

In T10599#178637, @mbohlender wrote:

I like the general direction. It gives a very clean first impression.

Some issues:

1. affordance There is no indication that is is possible to write something under the subject line. I am afraid some users will try to fit their whole email in the subject field. Then again, once the press "Enter" it should all become clear. Some placeholder text for the body could solve this issue.

I agree some sort of placeholder should at least be there.

1. minor inconsistency Once you are in the body ediror, and there is no text, I would expect "backspace" to get me back to the subject line (because "Enter" brought me to the body editor)

That's not what I would expect from how e.g. forms behave.
But we could try it I suppose (I don't have a strong opinion on it either way).

1. waste of screen space: This is probably my biggest concern with this design. The margins become really big when the window is full size. What was your reason for having the maximum width fixed?

One of the problems in the current design is that the editor gets very wide,
but you end up writing text on the left side only. This may be especially bad for me with a wide screen
and applications in fullscreen mode most of the time, but I think it applies in general.

The other point is that there are two kinds of people. The ones that manually line break text, and the ones that don't. Manual line-breaking is not great, because it will break if the line length was chosen too long by the author, resulting in the viewer reflowing the lines that are too long, ending up in "choppy" text (you get a mix of long and very short lines). If you don't break it manually writing becomes a pain, because long lines are a pain to read. There is a lot of anecdotal information on the internet suggesting a good width to be anywhere between 60 - 80 characters, so I aimed for 70.

So the reason is that the editor now is fixed width (aiming for roughly 70 characters), and everything else therefore is whitespace.
This also results in the editor being somewhat centered instead of being stuck to the left,
which I think is important because we don't use windows. If you e.g. look at what Apple does,
when creating a new editor, then they just pop-up a new window with a reasonable width for writing,
horizontally centered on the screen, yielding a similar result.

Also, I like the calm look for writing.

1. attachment placement that's something we might want to look at. It's not wrong. But we might be able to do better.

Sounds good. I'm also wondering how well the toolbar placement works out if it grows some,
and how much it will conflict with the otherwise very clean look we have now.

All in all this is awesome. It just needs some polishing.

Cool =) Then I think I'll just merge this so we can take it from there.