Some thoughts drew me away from making this app:
1) Something I would call "entry points" for this app. It should be released as iPhone and Android app and preferably some public JSON API to record events.
I assumed I won't be able to validate the idea if I won't provide enough "entry points" (apps, web, API) for my end users. They cannot be blocked with single entry point (web UI). Of course you can start small and grow organically (design iPhone app when users really need it).
2) Events (check-ins) are rarely "single points". They can have extra meta data (duration, location, whatever). If I track events with sentry or with GA, it collects many many extra data for me. If I would measure something with my app, I want to have extra dimensions and analyze how it affects results. It makes both UI and backend significantly more complicated.
I noticed that I could do some low-level open source (like charts library) or SaaS (like scalable BaaS for collecting events) instead to enable community to create useful products.
Again, you can start with basic events (check-ins) and expand later.
3) User experience issues, especially human computation and other trends. I think that only some small fraction of geeks want to explicitly track some events. I think that conducting experiments at scale should happen like in Duolingo or Recaptcha. I know that this was not the main point of the app. I just thought that I can do it better with gamification or advanced UX (like Duolingo guys).