In the new Gmail there is a huge amount of unused space, even in the "Dense" view. This results in less emails on the screen at one time in the Inbox (I use Priority Inbox so maybe it works great in regular mode, but it should work in both). The might have been trying to avoid information overload but they achieved information underload.
Now the compose and search buttons are enormous and the only colored elements on the page. Eyes are drawn to them in a way that each competes for attention and eventually you end up at the Gmail logo. Great for Google branding(? I already use Gmail), not so good for me because it hurts my eyes and distracts me.
Next, with everything being white with a few grey highlights, it is unclear where each section of the page begins and ends. Some things start to scroll while others stay static with no indication of what will change. The browser scrollbar takes up the whole size of the browser window but only part of the screen scrolls some of the time.
Just a few of the issues I had.