I had an idea very early on in G+'s life that it'd work great as a sort of bookmarking tool (nb: for myself, not necessarily others). I wanted to create an extension that I could use to "post" articles to circles I created (e.g. tech, or food or whatever) when I came across an interesting article.
I've tried other bookmarking services in the past, but never had the discipline to keep going (or I forgot). I figured because I use gmail frequently, and am always logged in, it would work well as a place I could aggregate things I wanted to read, but didn't have time to read at the time. It would also have the added benefit of being a list, or collection of things I found interesting/relevant that I could then share with other people. Could even have communal bookmarking with my closest friends.
Alas, I haven't opened G+ in months.