Ian Mackay on discovering applications in the Mac App Store:
I believe people have one of three things in mind when they are looking for an application to use.
Suitability for a given task. For example: “I want to write a novel.” “I want to create a birthday card.”
Similarity to an existing application. Some applications set a paradigm to which all others are compared (e.g. Microsoft Word, Adobe Photoshop).
Inclusion of a specific feature or design pattern. Usually one that the person uses a lot, so it’s a big time-saver.