This blog is not about Openness but given its relevance these days, I´ve formed my own opinion about it. It has been showing up on the radars of many for a while now and is related to a wider context. It is a central aspect of how people are working and might well be the defining characteristic in professional practice in development for the following years. The big boys have noticed it too.
In broad terms, Openness is the quality of a system being open. For our purposes, this means the disappearance of previous divisions in knowledge and practice. A large, heavily vertical institution tends to be less open than a small one, where everybody has a larger spectrum of roles, for example. In itself, this already signals that Openness is somehow related to the simplification of work. But we will get there in a second.
Openness manifests itself in organizations being more transparent and closer to their audiences; in knowledge becoming available to more people other than the specialists; in a bigger involvement from the masses in policy-making and so on. In a deeply cynical XXI century, this can be a bit of a surprise.I´m a bit of a cynical person, so I doubt all of this has simple “good will” roots. 
So where does it come from? Did just everybody decide to be cool and invite others in?
Openness follows a natural tendency of artificial systems to mimic natural ones. Artificial systems (e.g. formal organizations; companies and so on) have set rules and procedures (dynamically changing, by the way), whereas natural systems (e.g. biosphere, society) have frameworks of operation.  The biggest difference between the artificial and the natural, in this light, is who sets the rules and who follows them. When it comes to the artificial world, the prevalent tendency of simplification and modelling of reality results in a centralization (even when systems are said to be distributed, by virtue of their simplification, they leave out stakeholders) of decision power. Just as natural systems are highly specialized and adapted, artificial systems, man-made creations, tend to be more broad. No matter how specific an artificial system seems to be, I think that is all a matter of scale and perspective. A km is nothing in astronomic terms.
The problem is that the world has become more complex and less forgiving, so those who had the monopoly of knowledge and power (make no mistakes, somebody is likely to hold it still. Utopia is THE horizon), scrambled for ways of recruiting expertise and insight from those who had been out of the loop and could have something fresh to offer. This has happened both voluntarily and eagerly as well as cautiously. Open source software is an example of the first, the release of criminal investigation data to nab a wrongdoer is a case of the second, for example.
So, as the world gets more complex, segmented views of it no longer serve the purpose of ever increasing progress. Operating at a percentage of its potential capacity, society organized itself to correct artificial divisions.
Because it creeped up on us, it was a just murmur for a long time, but the present bias towards Openness has been raising steadily since the days of Industrial revolution, when industrialists shaped the school curricula to create a society of specialized workers. Although it was an artificial division of labor, it was achieved by the democratization of access to education. In the short run this is somehow ironic, but with perspective, it becomes more evident.
In the past few decades, we have been building the infrastructure to support Openness (e.g. education, communications, transport, etc) and now, our generation lives with free communications, easy access to independent publications, free and detailed information, freedom to associate, funding platforms and etc (this list goes for ever). It has never been easier for so many, to affect so many others, independently.
Openness, by dissolving previous walls, allows for deeper self-expression and realization and suddenly, we are all specialists in our own things. Designers are working around this principle.
Much like a theoretical terra-forming process, human action has steadily inched towards where we stand. Our society was constructed and Openness seems to be, from here, the natural balance of things. 
 In an interesting twist of this, US anti-terror strategy post 911 was to mimic the headless network of independent terrorist cells from al-Qaeda. Fascinating read at Wired.
Basically, the distribution of decision power is a sub-trend of Openness.
 For the most part, Openness is considered a good thing. This is, naturally, open for discussion. Yes, pun intended.
 In nature, and we are not talking about about animals and plants alone, conditions for survival and demise of a participant (e.g. a certain type of social role, or some cactus in a desert, or a star in a galaxy) vary widely. All this happens accordingly to the specific needs of said participant (e.g. the relevance of that social role; ways of collecting and storing water; availability of fuel).
 Because civilization is no static system, I’m not suggesting the end of war and famine, misery and overall jackassery. But if you look at nature, it seems that this is the way things should be.