If you're seriously talking about what it takes to be #1, then I don't think there is any one "killer feature". Any one feature, no matter how great, is overwhelmed by ecosystem, OS and hardware considerations. To pass iOS and close the gap on Android, I think MS needs to focus strategically on a three big things:
- Expanded and improved interoperability across MS products
- Continued refinement of a WP approach that is a hybrid of the iOS and Android approaches
- Increased marketing and promotional dollars behind the system to overcome carrier exclusives
If they can achieve real interoperability across Windows Phone, RT, 8, SkyDrive, Xbox devices and Xbox services, Office, etc. then the whole ecosystem becomes incredibly appealing to hundreds of millions of users. I'm not talking about your phone running x86 programs, but just having enough architectural overlap so devs are essentially able to write once, publish everywhere. Phones, tablets, laptops, desktops, TV's basically function just as screens, as windows that provide unencumbered access to and use of all your stuff. We see this in bits and pieces now, but there are just way too many gaps and limitations. Though this is obviously MS' vision, they have yet to demonstrate that they can really nail this the way Apple did with Apple TV, or Google does with Google Now or Chrome.
In regards to the phone experience, I think they need to split the difference between Android and iOS. They already have a For the people above who suggested just running Windows on a phone, I say you're crazy. They tried that with Windows Mobile- and while a lot of people liked it, most folks simply do not want to manage a full-blown Windows OS. They want "devices"- secure, fast, easy to operate and maintain. I think MS would be wise to open things up a bit to allow a little more customization and to give 3rd party apps greater access to core features and to each other. But recreating Android or full Windows is not the path to mass appeal.
Lastly, the crazy proliferation of Nokia devices this past year was amazing to see, but ultimately a drawback. Make 4 or 5 devices max, and put everything behind them. Unless or until MS somehow gets a huge hit, the only way to overcome the issue is by putting a lot of money in front of carriers. The upfront costs would be significant, but consolidation is the only way to maximize efficiencies on design, assembly and marketing (and the surest way to build an ecosystem of accessories that helps attract buyers and propel the platform). On the production side it is much easier to match output to demand; and you can improve processes more quickly to make assembly less expensive and to identify and fix issues. You reduce consumer confusion and angst (should I get the 920, 925 or 928? What if I try to get an unlocked version from another provider?? Does it support the right bands, will it work on this network, will it get LTE or 4G or HSPA+??? ). You let your marketing work against every potential buyer instead of just the ones on a particular carrier.
These are not quick fixes, or killer features- but no fix or feature will propel WP into first place.