What's actually "defendable" about web startups these days?
_ngw_: The only answer is the culture behind a decision. If you're doing things right you are following a "plan" and solving a problem, a feature is your catch about a possible solution. If your competitors copy it, you did your work right, it means that you have a good knowledge of your userbase and generally of the domain.
The problem will never be a competitor copying you, it's if a competitor implements your next feature before of you and the solution you're coming up with is the same.
At that point, you're not managing the rules of the game anymore.