Google has kept tight control over the product. Where-as Twitter grew from a diverse range of different products doing different things with the API and progressed towards a more authoritarian model, Google+ has been tightly controlled and normalized an experience. There's less room in the Google+ ecosystem for interesting things to work their way in.

There is a Moments API for external applications to post specialized content into the feed, and there is a read-api giving an Activity Streams centric feed.

Conversely, perhaps it's that we're less likely as a civilization to try to push things to interesting uses. Quake1, Quake2 and Unreal Tournament mark an exemplary period where dabblers came along and tried to push the systems, tried to find interesting and creative twists. Twitter was similarly buoyed into popularity, but today we see polished, final games- games are no longer moddable, would-be hackers simply license and leverae the engine, not a complete game to hack from, and like we see movements and projects such as IndieWeb, Tent.io, app.net, Salmon, ad-infinitum saying we must do it ourselves, rather than focusing on creating a rich dynamic around something that is.

Suffice it to say, my view is that anything that attempts to stand alone is bound by very low limits, and I'm not sure where G+ needs to go to overcome it's limits of being merely itself.