The key concept here is about security.
Imagine you could compile your web application/website into an application that has nothing else but the required network drivers and minimal hardware drivers needed to execute. That is (as far as I can tell from a brief look) what Mirage does. Using such an approach there's nothing for hackers to log in to - no shell to use, no other insecure cruft on the system potentially leaving holes. The attack surface of such a system is extremely low relative to putting fully powered and configurable interactive operating systems onto the Internet.
Hackers are showing that almost everything is hackable without expensive, constant, expert attention to security of every aspect of your systems. Even then zero day exploits expose systems within seconds or minutes of the exploit becoming known.
This is not to say that such systems cannot be hacked, but rather to say that in such an approach the attack surface is dramatically lower than in current approaches to deployment.
Perhaps more likely to change the way that people think about the concept of web application development in the longer term is that the application/server allows for the possibility of "high resolution server instances" in which a virtual machine can be started in milliseconds, perhaps to service only one query, and then vanish again. Instead of renting your instance from Amazon that runs per hour and does nothing most of the time, your infrastructure will be truly scalable with nothing running at all until its needed and then precisely as much computing power will be used as is required to meet the inbound demand. No longer will you need to "start a web server" which then chugs and rumbles into existence as all its various unneeded-at-most-times processes start. Instead, connect your application/server AMI to the Amazon spot instance marketplace.