Most non-developers:
- don't know what APIs are available (and suggest stuff that that a stock OS iPhone simply can't do).
- don't know the SDK rules (and suggest stuff that would violate the developer agreements).
- don't know what would be too hard to create (some video games require a team of a dozen devs and artists, or more, working for many many months. Do you have the money to pay that many people a decent and competitive wage?)
- don't know enough marketing to know what would sell to enough other people (there are lots of cute apps at the bottom of their categories, did the dev even make back his/her $99?)
- suggest stuff that's already in the store, but they just haven't found yet.
Many of the few good ideas left is stuff that other devs are already working on.
And then there's stuff that would only be good if you could get it into the App store first. If you read about it on a web site, you're probably already too late to start.
ymmv.