There is nothing groundbreaking about what I am about to say... but: it is easy to overlook how simple and powerful making purchases with a single-click is. You can download music, movies and rentals with a single click. No credit card number.
No lengthy registration / shipping addresses.
Nearly every book title is available via Amazon or iTunes through a single click. The content is delivered wirelessly in seconds.
The same holds true for video games and, now, an entire world of software.
In fact, Mac's App Store is what hit this point home for me: downloading software applications for your desktop / browser is really a powerful experience. You can now buy software with the same one-click, light-weight process that music, books and mobile apps are purchased. And it is delivered over the web and instantly.
It is hard to describe why this is a unique / 'cool' feeling... after all, I've purchased software online before. But, until the Mac App Store, it had not been so effortless, painless and fun. And it's thanks to both Apple and the developers / cloud movement, which allows us to access myriads of content (much being lightweight) so rapidly.
And it's just the beginning. Other major app stores will emerge (ie Chrome, Amazon) and Facebook Credits will play an enormous role as it will enable all websites to become their own one-click storefront.
When you click through, you land directly on the Happy Island page and realize how hard developers must fight for user actions. Much of the viewable screen is taken over by a request for your email address and then for a positive review (particularly common in iPhone Apps). Considering Happy Island has 12,000,000+ actives, you have to believe they are savvy enough to have determined that these placements are more effective upfront than after a player has become engaged in the game: