Ever since I’ve been building web pages, it has been gospel that your logo (top-left, of course) should be a hyperlink back to your homepage. Jakob Nielsen’s seminal Homepage Usability recommended as such (with the caveat that the link should not be live on the actual homepage), and it is almost a subconscious action to wrap that
IMG or text in an
A tag and link it to “/”.
Strangely though, Google’s most recent refresh of their apps have dropped this two-decade convention. None of their properties – GMail, Google Docs, Google+, et al – make the Google logo into a link; now if you want to return to the default view of the app, you must locate and click the much smaller “Home” link (or, in the case of Google+, the even smaller Home icon in the navigation bar).
I suspect that the reason is an attempt to train users that the top light-grey bar on all Google’s properties is not tied to the app you are currently using, but is instead a persistent area present across the entire range. The right-hand end of the bar is now home to your G+ notifications, avatar and share link, so perhaps they hope that by dedicating the top 100px of the page to their social network they can encourage adoption and/or use. I think that the loss in terms of usability and user expectation is disappointing.