How do we prevent a large competitor from just implementing our feature?

In software, even if you patent it are you able to defend it? Look into the cost of litigation insurance. If all you have is a "feature" then perhaps the problem is the business model (insufficient differentiating value) and not the feature per se?

I'd point out that first mover only works under certain circumstances and fast followers, especially the chinese hordes is impractical to defend against. See my post on Dunedin StartupWeekend on indian rope trick.