I think anybody developing open source / collaborative commons software that targets non-technical users could learn some tricks from streetcomplete. It has managed to lower barriers to entry and seems to get many people engaged in what is quite a technical area. Some UI expert might be better able break it down but here is my braindump of some key factors:

* at a basic level, generally smooth handling of drawing of a legible map and basic UI functions (esp. compared with some other foss openstreetmap apps, no need to name names :-)

* considerably narrows down the map editing options. by presenting controlled choice lists the novice editor feels "safe"

* uses simple and innocent gamification (scoreboards, "unlocking" websites etc)

There are ofcourse glitches and indeed a keen openstreetmap enthousiast may "outgrow" it relatively quickly but assuming the devs get further support and build on the initial success it could be a reference point for highly usable open source apps targeting larger audiences