Browser support. We've got major internet sites on board waiting to deploy it, but despite some early indications of support, the browsers have not been forthcoming. We even tried writing the code for them, but to little avail.

My sense is that Google is all-in on Certificate Transparency right now, and doesn't want anything to distract from that. However, I think a successful CT deployment is a long time coming still, and TACK could have been out two years ago. My sense is that Mozilla is just understaffed and/or waiting to follow Google's lead in this area.

Ultimately, this is the frustrating thing about trying to improve the state of secure communication today. Most of the levers are in the hands of the browser vendors, which makes it difficult for those of us on the outside to help iterate things forward.