The new GMail for Android UX sucks. I mean... it's really awful.
They've replaced the checkboxes next to each message (useful) with sender images (gimmick), or, if there is no sender message (i.e., everything that's not a G+ contact - so, every newsletter, receipt, order confirmation, etc. you'll ever get), a big colorful first initial (completely useless waste of space). This image then acts as if it were the checkbox that used to be there (confusing) for selecting messages. You can turn off the images, but you don't get the checkboxes back; you can only tap-hold to select multiple messages, though this isn't mentioned anywhere, you just have to guess.
They've gotten rid of the delete button (why?), and moved it to the menu.
If you have no messages selected, pressing the device's menu key gives you the menu. However, if you do have messages selected, the menu key does nothing, instead you must tap the menu button that appears at the top-right of the display. It's not there if you don't have messages selected.
Once you're viewing a message, there are two menus: one when you tap the menu button, with 90% of the options in it, and another at the top-right gives you just two options, forward and reply-all; this almost makes sense, except that it uses the same, standard "here's the menu" button that's used on (some) other screens as the *only* available menu.
In the message view they've also gotten rid of the delete button (to match the annoyance of the message list, I supposed).
There is also a new "label settings" screen that's fairly mysterious; I assume it applies to the current label, though this includes "Inbox", which - while I understand it's treated internally as a label - I think most users don't think of as being a label in the typical sense.