Recently I took a survey asking people what they wanted from a mobile app in general and put the results up here. Although a lot of it seems like common sense (which it is) it gives you a good jumping off point from which to start.
A crowd-sourced app in particular is going to need good UX as it requires the user to do more than just use the app, they have to participate in it. From my experience creating this type of app you'll need to give users a "reward" for participating (points, prestige anything like that, much as Stack Exchange does) and make it easy enough for them to do that they won't fall at the first hurdle.
As a rule of thumb:
- Only ask for information you need, lots of inputs will put users off
- Make it easy for them to fill in the information you require by using the right type of input (for example if you want their email address then use an email field, this will swap the keyboard they see in the app to the email specific one)
- Give the user something for their effort (be it a badge, some points or a tag that tells other users how awesome they are)
- Let users chose to do things themselves. No pop ups or banners asking for users to do things, just make them aware of the ability to add/change/edit etc something and then leave them to decide if they want to.
- Minimize the amount of personal data users have to enter. Unless you really need it don't ask for personal data, it makes users wary and less likely to want to contribute as they wonder what you want that information for.
- Add a social aspect. From logging in using Facebook to posting to Twitter that they have done something to your app this gives the user incentive (and in the case of logging in it's much easier for the user) and allows users to interact with other people via the app. It also has the added bonus of marketing your app for you.
- Don't expect every user to want to participate. Crowd sourcing is great, it gives a real human aspect to an app but not every user that installs it is going to want to add/change/edit things, some are going to be using it for reference, don't make them feel guilty or hinder this.
Unfortunately there doesn't seem to be much literature which is specific to crowd sourcing in mobile apps so checking the usual places such as Smashing Magazine's UX Site is probably the best thing you can do.
The main thing to do is apply common sense and ask yourself: "If I was going to use this app what would annoy me, and what would inspire me to contribute?"