I donno but I think this is Question one:
Does something like this exist already and are people actually paying to use it? (Unless of course you're selling a (great looking) service for all the other questions).
Sometimes your "killer" idea might be selfish or for a really small niche, too. It may not be a successful business venture, but it could be the killer app for you, friends and family, or specific types of users. For example, in another blog post of mind somebody older commented on his desire to have an app that simply uses the camera to automatically enlarge what he's looking at to help him read. And if you're interested in learning or building things then perhaps working on your own personal project will be super rewarding.
Lots of people do not know what they want. Hardly anyone knew they "wanted" FB because nobody actually thought they'd be needing it. Less alone pay for it.
If it's really "killer" (not as in: "I'm creating a social app where people hunting cows with bows can use their smarthpone to share videos of arrows piercing bows in real-time") then people may not know they'll actually want this.
I don't disagree that having other buying apps using your idea is good in that it shows there's a market.
But then you don't "Have a great idea". Because you're idea simply is "My idea is to copy another product".
Not that I have anything against it: it's what I do for a living : )