Call me pessimistic.
Every viable web startup idea that I can think of has already been done by other startups who have been there many years before. Many of these startups' products look very polished and usable, that I can't begin to think of how to make a substantially better product that would encourage people to jump ship onto my product.
For example, one of my ideas is a repository hosting service. But why the hell would anyone use my service when there's already GitHub/Assembla/BitBucket?
I'm thinking of just copying other startups but charge less as a possible strategy.
Is this a good idea?
Don't do this.
Your job is to provide something people want that you can scale. 99.99% of the time somebody will be able to point at what you are doing and say "That's already been done!"
People who say this have no idea what they are talking about.
A business is about a hell of a lot more than some unique, creative idea. In fact, I think it's almost a reliable metric to reverse this thinking: what's been done to death that other hackers wouldn't like? To me if you want some kind of system for ideas, that's where I'd go.
But the big-picture ideas are almost always worthless because the devil is in the details. The "why the hell would anyone use my service" is spot on. The answer will never be something like "I have a unique idea" -- it'll be a lot more complicated than that.
I consume a lot of things, and I never make consumption or purchase choices based on who actually came up with the business concept or how many other folks are doing it.