Several good discussion points have been raised in this thread; reliability, latency, longevity etc. all of which are important considerations.
There is one aspect that has not been covered that is equally important; the interaction of people and media. One of the basic principles that make tape cost effective is the ratio of media pieces to read/write stations or tape drives. The higher the ratio the more cost effective tape becomes. However, this is just exploiting one attribute of a dual set of attributes; removability and portability. Removability is good, it enables the overall cost to be lowered; portability is bad because it introduces the protein robot or the protein based automation platform, the human.
When we humans touch things we open the door to errors. These errors are numerous in nature and have the real potential of making the concerns of any one of the technologies referenced in this article as quite trivial in comparison. When the attribute of portability, the removal of media from the automated tape library, is exploited we invite the chance for error because the data/information has been removed from the control of the system; we are entrusting the data to the human process and if history teaches us anything this will lead to serious, costly and inevitable problems. Remember this: http://money.cnn.com/2005/06/06/news/fortune500/security_citigroup/. And it doesn’t just apply to tape; the attribute of portability applies to USB drives as well; http://www.thestar.com/news/gta/2013/10/07/health_information_of_18000_people_stolen_in_peel_region.html. Lastly, in this story you will note that encryption was referenced as a guideline that was not followed in the case of the USB drive. The encryption of data on portable devices does not solve root cause; it may prevent unauthorized access to the data but the data is lost to the entity that put it on the portable media to begin with – fix root cause.
In order for us to manage the Peta/Exa/Zetta bytes of data heading our way we must keep it all under the control of the system or we have no chance of success. Cold storage based on HDD spin down that enables density and lower running cost metrics, offers hope for management under policy-based systems control because it removes the opportunity for the protein robot to inject an error prone process.