Thinking about this more, the problem isn't that list apps are an underserved market, but rather that they're an overserved market. Making a list app is easy -

Thinking about this more, the problem isn't that list apps are an underserved market, but rather that they're an overserved market. Making a list app is easy - so easy that "make a list app" is the glorified "Hello world" used to demonstrate real-world code for lots of Javascript frameworks and other language libraries. Successful examples are vertical, not horizontal. The obvious ones mentioned here are Trello and Pinterest, neither of which are "list apps" in the standard sense. Other ones coming to mind are Buzzfeed-style click-bait, of the "Here are 17 stupid things! Number 4 will shock you!" variety - but those aren't so much lists as bait technique.