Could this be an iPhone 6 killer? I'm thinking about not upgrading to an iPhone 6 when it comes out if it doesn't have a larger screen, and wider not just taller like they did with the iPhone 5.
About as likely as the Kindle becoming an iPad killer.
Fire Phone has good hardware and some cool features, but it won't have the things that are more important than that: ecosystem, large library of quality apps, refined OS, integration with iDevices, lots of accessories, best warranty and support, better built materials, voice assistant, available on more than just 1 carrier, etc...
The larger (not just taller) screen is pretty much certain at this point with the leaks that have been seen.
The Fire Phone looks like it could be a good first entry, and have the potential to grow into a serious competitor though.