Companies are constantly releasing phones that analysts are quick to call ‘iPhone killers’ or gamechangers, thinking that another company will take over part of Apple’s stronghold in the smartphone market.

They said it about the Motorola Droid and they said it about Google’s Nexus One. But this time it isn’t a phone – it’s an entire mobile operating system (OS). And it’s not from RIM or Google, but from an unlikely contender: Microsoft.

The company showed off what it calls Windows Phone Series 7 at the Mobile World Congress (MWC) in Barcelona. The OS is a complete jump from its previous forays into mobile phones, being nearly unrecognisable from Windows Mobile 6.5, its predecessor that is currently available. The new Windows Phone Series 7 isn’t set to release until later this year and all of the specs aren’t available, but one writer at Gizmodo who handled a new device already declared that Microsoft has “out-Appled Apple” with the new user interface.

The new OS is said to focus on information, rather than apps, and features an interface very similar to the Zune HD and will supposedly have access to the Zune marketplace. Users scroll side-to-side and up-and-down to access information, all of which is contained in ‘hubs’ such as people, pictures, music and games. Like the iPhone, the new OS relies primarily on touch and will likely not contain multi-tasking.

Microsoft’s intro video for the OS at MWC, posted on TechCrunch, took big digs at the iPhone and its single-app platform, calling it app-focused and part of a “sea of sameness” in mobile devices.

Though the outlook on the new OS has been resoundingly positive so far from most bloggers who have seen it in action, it’s too early to tell if Windows Phone Series 7 will actually upend the iPhone’s place in the market. And with the next iPhone rumoured to be in the works, it’s still anyone’s game.