Time for me to add my 2 cents. My ISP 'upgraded' last night from v15 to v16 without warning. My users were just a wee bit on the shocked side when everything changed. And by everything, I mean...
v16 is slower.
Being 'responsive' does not translate into faster.
No support for older browsers.
Again, most likely the result of becoming 'responsive'. What about those individuals that can't run the lastest whiz-bang browser, even on their mobile device.
Too much screen real estate wasted.
You state that it is sized for screens 1366x768. With that in mind, it should be painfully obvious that vertical space is scarce and wasted space should be kept to a minimum
No mulit-column format option
No confirmation of delete when selecting multiple messages
Not only is there no 'do you really want to' prompt, there is also no indication that it's being done.
A message must be clicked to open instead of the simple press of the 'Enter' key.
Who cares about a pretty picture on the login screen?
It adds nothing to functionality, but DOES consume data on mobile devices.
The message date is way more important than the message size.
While I do use the message size on occasion, I would much rather have the message DATE on the left instead of all the way to the right margin of the window. Then again, there's also no valid reason why the both couldn't be on the left, as in
(date) (size) (attachments) (flags)
There is less wasted space and the important stuff is all together and it isn't missed by being thrown to the edge of the window.