There are buttons that are not obviously clickable, others with no help on what they do, and features that are hard to find. The profile display is completely confusing, especially with its double right scrollbar and no way to make it go away. Menu options for posts don't appear unless you happen to click on a blank space of the post. Why anyone would ever think to do that, aside from accidentally, is not clear.
One of Google+'s most innovative and useful features, Circles, has been progressively downplayed to the point where it's now practically invisible. I bet most people don't have a clue how to establish circles much less use them. The use case for Collections remains ambiguous.
And just because people are using tablets and phones a lot doesn't mean it's a good idea to hobble the desktop functionality. Make responsive interfaces that are appropriate for the different usage techniques of these devices. Maybe it's time to finally give up on Google+.