I can point to mistakes I made such as the higher posting. In our case some extra numbers are sent with each event and the values of those numbers are interesting. (One example is the volume level of the device.)
With Google Analytics where they term this custom variables all you get to see is the average value which is spectacularly useless. Mixpanel would show us the distribution, but only for a particular event type. We have about 20 different events, so working it out across all 20 would be too tedious.
Today we only use mixpanel as a receiver of events. We export the analytics data from them and then work on that locally. Unfortunately they only provide an export every 24 hours. We do not use any of their other functionality although we did try.
For mobile app analytics you need a client library in Java (Android) and Objective C (iOS). The client library needs to record analytics events into a SQLite database, and then periodically try to upload them to a server (you don't always have connectivity). Attention must also be paid to things like roaming (do you want to burn user's data, it can be expensive in many parts of the world). You also need cleanup (eg if you can't send data for many days then you'll likely want to discard it). You'll want to make sure the client plays nice (eg not creating lots of threads and causing constant wakeups). It should also supply platform information since you'll want to analyze versions, screen sizes etc. There are various other little details that matter on the client.
On the server side it needs to correctly cope with data arriving days late, with "incorrect" time stamps from clients. And you'll want to easily do pointy clicky through the data as you'll have some common questions such as what are the most prevalent platform versions and devices and how does that correlate by country.
