From the video: "Our servers store metadata about your messages, but not the messages themselves"

It's difficult for me to invest the time (and effort) required to use a service like this, when that investment could easily just go away when the service is pulled for whatever reason. Or, if their servers are down will I have robust access to my mail client?

One of the nice things about old email is that it is portable. Metadata on 3rd party servers seems to cast this positive aspect about email aside.

Some aspects of these new emerging services are novel, specifically the UI and layout of features. As many have pointed out recently, similar functionality has existed in mail clients for years. However, not every email user is aware that their use of email could be made more productive just by utilizing the tools already in their email environment. I would guess these mail client-services are catering to that market -- the less technically inclined. Consequently, these customers are less likely to know or care about the implications of spreading vital work-related or important data across new and potentially volatile services.

That said, I do support nice-looking and more usable email clients, just not at the cost of dependence on a 3rd party service.

I've already looked at Postbox, made by some of the previous Thunderbird developers. http://postbox-inc.com/ I'd have to try these to even come close to figuring out what to use.

But the idea of SSO for email is a bad one, in my opinion. Email, more than my user account on a site, defines my online identity—and therefore needs the greatest protection.