Internet / mobile consumer propositions can be constantly changing, and ambiguity can be a great driver of the creative product development process. In investor pitches though, you need to freeze the brain storming and present one consistent, stable product.
Investors need to make 2 leaps: understand what your product is about, and get a sense for the product you want to offer. In 20 minutes, you can't introduce 3 variants, discuss 10 other cool ideas, leave that for the follow on meetings.
These type of app ideas are very hard to describe in bullets or even pictures. The best way to get things across is to create a very specific user case example and describe a sequence of events, interactions between users. You can alternate between images, mock ups, etc.
The priority of all of this is to explain and show things, "pretty" comes in second. Sometimes investing a lot of energy in making screen mockups with buttons and sliders actually makes the app idea harder to understand. To show that you have design skills in house, you could take one app screen and turn it into a pretty mock up.
A side effect of this effort in an investor presentation might be that you get more clarity inside your senior team what your product should look like.
Image by Nicolas Henderson on Flickr