When you look at Windows Phone or BlackBerry 10 it's easy to see the well-known iOS and Android apps that are missing from their respective app stores. Even with Android, you can look at popular iOS apps like Tweetbot, Fantastical, or Letterpress, not to mention every single one of Apple's apps like GarageBand, Keynote, and iTunes Store, and notice their absence from Google Play. With the iOS App Store, however, it can take a little while longer. The Google apps are mostly there, as is BBM, and many Microsoft apps as well. So what's still missing?