As with all good things that must come to an end, the pure spartan simplicity that defined Google at the start has begun to cede ground to the ever-encroaching clutter.
Usability trouble comes in various guises but could be summed up in a word as "inconsistency". If I had a mantra for usability it would be "meet expectations". Which is impossible when your products set conflicting expectations. With all the additions, acquisitions, and new beta projects coming out of Google now we're starting to see the inevitable usability dragon rearing its ugly head.
It happens, for example, when you click on "My Account" in one application and it takes you to the app's dashboard, click it in another application and go to your Google account preferences. I don't want to have to remember that two identical-looking links in identical fonts in identical places on Google apps have completely different functionality.
Dear Google, please fix the inconsistency.