It is fun watching all the digital chatter about blockchain at the moment. There is wild stuff at both ends of the spectrum. I.e. "It is rubbish. Will never fly. All hype. Nothing new here. Forget about it." on one end and "Sliced bread has finally met its match! Lets appoint a CBO (Chief Blockchain Officer)" on the other.
Here is the really important bit I think: The blockchain shines a light on an interesting part of the Noosphere. The place where trust in information is something than can be established without needing a central authority.
That's it. Everything about how consensus algorithms work, how long they take to run, how computationally expensive they are, are all secondary and the S curve will out (http://en.wikipedia.org/wiki/Innovation). I.e. that implementation stuff will get better and better.
Unless of course, the proves to be some hard limit imposed by information theory that cannot be innovated around e.g. something analogous to the CAP Theorem or Entropy Rate Theorem or some such.
To my knowledge, no such fundamental limits are on the table at this point. Thus the innovators are free to have a go and that is what they will do.
The nearest thing to a hard limit that I can see on the horizon is the extent to which the "rules" found in the world of contracts/legislation/regulation can be implemented in "rules" that machines can work with. This is not so much an issue for the trust concepts of Blockchain as it is for the follow-on concept of Smart Contracts.