Chromecast could be the greatest thing ever* (*when the SDK opens up)
Here's what I think:
At some point within the next six months, Google is going to open up the Chromecast SDK to all developers. At this point, they're going to start encouraging all developers to support the Chromecast - not just developers of media apps, but any app that displays content.
"Chromecast mode" is going to become another form factor that apps should support, along with smartphone mode and tablet mode. Since Android already uses a system of responsive design where apps intelligently redraw themselves for different form factors, this shouldn't be too difficult for developers to add. However, the best apps for Chromecast will not simply rearrange their layout for a television, but will think cleverly about a how to provide a dual-screen experience that takes advantage of both your smartphone/tablet and your TV screen at the same time.
What makes this so powerful - which I assume is Google's plan - is that the Chromecast could end up becoming ubiquitous due to its crazy low price. If this is the case, then there will be an expectation that any large screen that you encounter will have a Chromecast that you can fling content at. And the Chromecast is super portable, so if necessary, you can always bring your own.
Some examples of apps that could benefit from Chromecast support:
1. Media apps: This is the obvious use-case for the Chromecast, and the "first wave" of Chromecast apps have all fallen into this category. We've seen streaming video and music apps, and local streaming apps like Plex. I expect that apps which play media directly from your phone (eg. MXPlayer) and podcast apps (eg. PocketCasts) will come soon.
2. Photography apps: Any app that is primarily about displaying pictures could benefit from Chromecast support. That includes the stock gallery, but also apps like Dropbox, Instagram, 500px, Pinterest, and many others.
3. Social networks: How many times have you been in a group of people, and someone says something like "check this Facebook picture of this guy I met last night!" or "look at what she said on Twitter today!", and then the smartphone gets dutifully passed around so everyone can see. With Chromecast support, you could just fling that content onto the nearest screen so everyone can see at once.
4. Casual games: You could use the Chromecast to duplicate the experience of a board game. Imagine a Scrabble-type game where the TV screen shows the board, and your phone shows your individual letters. Or a Texas Hold'em game where the phone shows you the the cards in your pocket, and the TV shows the cards on the table. You could have trvia games, strategy games, music games, etc. implemented in the same way.
5. Console-style games: For more "serious" games like Riptide or Shadowgun, perhaps it would be possible to display the graphics on the main screen and just use the phone as a touch-based controller? At least then your fingers wouldn't cover up the action on the screen. This would depend on latency on the Chromecast being low enough to work, so it might not be possible on this generation. Also, I kind-of hate touch-based controls in console-style games, but this might be a decent cheap alternative for people who don't want to shell out for a "real" gaming console.
6. "Office" apps: Eg. QuickOffice, Google Drive, Microsoft Office 365. How cool would it be if you could do a presentation without even having to open your laptop? You could just bring a Chromecast, plug it in to the screen or projector, and stream your presentation from your phone.
7. "Party" apps: The YouTube app already allows a group of people to simultaneously use the Chromecast and add apps to a queue. Apps like Spotify or Google Music could take this further and allow a group of people to build up a music playlist or even vote songs up and down, like the old iTunes DJ feature.
8. Others: There are so many other possibilities. Fitness apps. Mapping apps. Web browsers. Video chat. And perhaps entirely new categories of apps which haven't been invented yet, because they are designed from the ground up to have a dual-screen experience.
I think this could be really great.