Well I'll tell you, from the perspective of someone who managed a development team, what the big deal is. It's managing information overload.
Suppose you're maintaining an Android App. No problem, you just need to know Java and the Android SDK, which is very well thought out and amazingly easy to use. Oh, but now we need a server tier to our system. Well, you could learn PHP, but to keep things simple you stick with Java and go Java EE, which is not so hard to learn these days. You decide to use a RESTful interface, which Java EE supports well, so you don't have to learn the ins and outs of Servlets. Now management says you've got to run on the iPad and Blackberry as well, so you learn IOS and QNX -- I'm assuming you're a genius, so it's not that hard for you. Oh, and there should be a web interface for desktops. You could learn Java Server Faces, but you really need to learn HTML to do that anyway, and you have that nicely thought out RESTful interface, so you decide to go with HTML and jQuery or Dojo so you can serve up a slick Rich Internet Application experience. You also need to master several different application frameworks to provide things like mapping on each end user platform. But it's no problem for you, you're a genius.
Fine, but now think about what it would take to hire your replacement to maintain your software.
I once worked for a small app developer that over some years developed an app that looked a lot like what I've described above, although the specific technologies were somewhat different. The fact that the project had accreted so many dependencies didn't faze any of the five of us who worked on it. All of us either had advanced degrees in CS or more than ten years of experience in the field. We were very successful for our application domain so a larger company bought us out. But even though they'd spent millions of dollars to acquire the product they gave up on it within three years. It wasn't because of competition -- there was none. It wasn't because customers didn't want to buy the software -- they did. The problem was the new owner was never able to assemble a team that could master all the technologies needed to maintain and support the system.
This doesn't make HTML5 the ideal platform for writing Android, iOS, QNX or Windows applications. But if you had to target two or three platforms that coincidentally support HTML5, it's certainly worth considering these days.