Any time I review an app, I compare it to the baseline in the category. When I reviewed Fantastical for iPhone I compared it to apps like Calvetica, Week Calendar, and Agenda. Not only did I find that Fantastical had a better feature set, but it also delivered a smoother user experience. The calendar app category has been around for YEARS, and Fantastical had years to prepare to win. Ultimately as a developer you must aspire to be the best in your category, which often means looking to competing apps and making sure you can outdo them from the get-go — and *don't launch your app until you meet those goals* since any category only has a few real winners. If you can deliver, then you've got a "good idea." You have your work cut out for you.

To me what makes a good app idea is a good interface idea: the two go hand in hand. More often than not problems we identify (which aren't really *that* hard to identify) have HARDER usability problems that piggy back onto them. For example, you want to make an app to help people track their calories. Great. It passes the "idea" tests. And yet with this "idea" you've said nothing, absolutely nothing, about the *real* problem, the interface--helping people overcome the tedium of logging depressing nutritional data day in and day out.

In a perverse way, it's not your big-picture solution, but your *interface* solution that's going to get you your first round of press and give the app legs. And it will give you a story to tell about your app.

A good app has a clear message and purpose. The tools have to be unique and fill a need, or people will question why they need it or just stick with something they have.

But the message has to be simple as well, right down to your app's startup screen and empty states (emptystates.tumblr.com) and your App Store description. The "elevator pitch" is a cliche but it is absolutely true. You have to be able to explain your app's core purpose in a succinct way, just like you give the quick single-sentence pitch to new friends at a party describing what you do. The details and extra features certainly matter, but those can come later or in a different way.

Great responses. I think both Ellis and Samuel hit on a crucial point that I hoped would be brought out: in a crowded market like many categories on the App Store, the idea itself may be only marginally better than the competition (it may even include fewer features). The real genius of the app is in those micro design decisions that make it a better experience. At the same time, it is important to identify an idea where executing such an amazing experience will pay off, business wise, for the developer, which is why all these other points are important.

And David makes a great point. Apple has a really great practice of coming up with one sentence that describe what the app does and who it is for. So "Grades helps students see what grade they need to aim for on their next test." If you can't describe the core idea in a succinct sentence (and later, in a screenshot of a core screen), the idea is probably too complex.

Jeremy and David, this point reminds me of a great Steve Jobs quote: "A feature that users don't understand is a feature they won't use." So aside from the marketing consequences of having an ambiguous message, you've wasted resources adding those features because users won't use them!

We're entering a time with apps where most of it has been done before. A good app idea adds on to the competition's with some better features and implementation. A *great* app idea reimagines that product through design, and resets the baseline. I've also learned that good design (form and function) is great marketing.

We do need to expand on Jeremy's point about pay-off and other things. To be completely honest, I've seen very few apps succeed purely on the merits of their premise ("big" idea), but I'd put Instapaper and possibly Vine on that list, where you say, "man, that's a really clever idea for an app." (I can elaborate if you guys wish.) To reiterate, the overwhelming majority of successful apps I've seen have solved interface problems to obvious needs.

To expand on David and Jeremy's posts, you need to have an Elevator Pitch for the app. If you can't explain it succinctly in one sentence then you have no chance of attracting attention on the App Store where your only have 5 seconds to capture your customer's attention..

Alongside this, once you have your Elevator Pitch, producing other marketing material like your screenshots, trailer, website, social presence, etc. is now very easy.

To Jeremy's point, this came directly from Apple: Easy to use digital photo sharing for casual photographers
Pick few features most frequently used by majority of your users most appropriate for mobile users

"Your differentiator your solution for your customer."

To expand on Elia:

During a recent interview, Andrew Mason beautifully articulated a view I've always held to: "Technology products work best when they’re only one iteration away from what people already know."

Improving an existing behavior your audience already understands removes much of the work your elevator pitch needs to perform. You don't have to explain the usage, just why you make something familiar better.

Then keep making it better until you have something new.